Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)

Hodel, Florent; Macouin, Mélina; Triantafyllou, Antoine; Carlut, Julie; Berger, Julien; Rousse, Sonia; Ennih, Nasser; Trindade, R. I.

doi:10.1016/j.precamres.2017.08.005

Cited by 43 publications

(82 citation statements)

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“…760 Ma 15 , 16 , Morocco) experienced an intense hydrothermal activity that produced unusually massive, up to 5 cm thick magnetite veins 11 , 17 , 18 . A detailed magneto-petrographic study 18 of the hydrothermalized serpentinites hosting the veins showed that an intense iron leaching in the serpentinites by a Cl-rich acidic fluid provided the iron for magnetite precipitation. Both abyssal and tardi-orogenic settings were proposed concerning the involved hydrothermal event 18 .…”

Section: Resultsmentioning

confidence: 99%

“…A detailed magneto-petrographic study 18 of the hydrothermalized serpentinites hosting the veins showed that an intense iron leaching in the serpentinites by a Cl-rich acidic fluid provided the iron for magnetite precipitation. Both abyssal and tardi-orogenic settings were proposed concerning the involved hydrothermal event 18 . Here we provide geochemical data on the serpentinites attesting that a black smoker-type (abyssal) hydrothermalism generated these unique magnetite veins.…”

Section: Resultsmentioning

confidence: 99%

“…760 Ma Bou Azzer ophiolite 11 – 16 (Anti-Atlas, Morocco), hosting massive and well-preserved magnetite veins 11 , 17 , 18 . We confirm that these veins derived from a black smoker-type abyssal hydrothermalism 18 . This high-quality isotopic archive consisting of pure magnetite from the massive veins allows us to propose an accurate δ 18 O value for the Neoproterozoic bottom seawater.…”

Section: Introductionmentioning

confidence: 99%

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Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater

et al. 2018

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The evolution of the seawater oxygen isotopic composition (δ18O) through geological time remains controversial. Yet, the past δ18Oseawater is key to assess past seawater temperatures, providing insights into past climate change and life evolution. Here we provide a new and unprecedentedly precise δ18O value of −1.33 ± 0.98‰ for the Neoproterozoic bottom seawater supporting a constant oxygen isotope composition through time. We demonstrate that the Aït Ahmane ultramafic unit of the ca. 760 Ma Bou Azzer ophiolite (Morocco) host a fossil black smoker-type hydrothermal system. In this system we analyzed an untapped archive for the ocean oxygen isotopic composition consisting in pure magnetite veins directly precipitated from a Neoproterozoic seawater-derived fluid. Our results suggest that, while δ18Oseawater and submarine hydrothermal processes were likely similar to present day, Neoproterozoic oceans were 15–30 °C warmer on the eve of the Sturtian glaciation and the major life diversification that followed.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater

et al. 2018

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“…It is mainly made up of ultramafic rocks (serpentinites and few chromite pods), but also mafic meta-cumulates, meta-basaltic sheeted dykes and pillow lavas in a smaller extent (Leblanc, 1975(Leblanc, , 1981. The geochemical signature points to an emplacement in a supra-subduction zone (SSZ) setting (Bodinier et al, 1984;Naidoo et al, 1991;Ahmed et al, 2005;Hodel et al, 2017). Precise radiometric dating of an igneous event forming this oceanic crust has not been performed yet.…”

Section: Geological Outline and Previous Workmentioning

confidence: 99%

Intra-oceanic arc growth driven by magmatic and tectonic processes recorded in the Neoproterozoic Bougmane arc complex (Anti-Atlas, Morocco)

Triantafyllou

Berger

Baele

et al. 2018

Precambrian Research

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“…Ultramafic rocks host many important metallic deposits including those of Cr‐Ni‐Fe‐Co‐Cu‐Au. In particular, it has been shown that magnetite and magnesioferrite are closely related to serpentinisation (Ahmed & Hall, ; Awan & Sheikh, ; Eslami, Arai, Miura, & Mackizadeh, 2018; Gahlan et al, ; Gargiulo, Bjerg, & Mogessie, ; Hodel et al, ; Khedr & Arai, ). The serpentinisation of ultramafic rocks is a hydrothermal alteration process, with magnetite as one of the major products (Huang, Sun, Ding, & Wang, ; Klein & Bach, ).…”

Section: Introductionmentioning

confidence: 99%

Serpentinisation and magnetite formation in the Angwu ultramafic rocks from the central Bangong–Nujiang suture zone, Tibetan Plateau

Liu

Tao

et al. 2019

Geological Journal

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Serpentinised ultramafic rocks are widely distributed in the Bangong–Nujiang suture zone (BNSZ) and show close relationship with the regional mineralisation. Here, we investigate the serpentinised ultramafic rocks from Angwu in the central segment of the BNSZ, where the primary minerals include olivine, clinopyroxene, chromite, lizardite, magnetite, and chlorite. Based on a comprehensive analysis of the results from petrography, back‐scattered electron images (BSE), and energy dispersive spectrographs (EDS), as well as electron probe microanalysis (EPMA), the alteration process of the ultramafic rocks, including serpentinisation, can be divided into three stages: 1, formation of relatively Fe‐rich serpentine (Mg#, 0.82), with no precipitation of magnetite; 2, formation of relatively Mg‐rich serpentine (Mg#, 0.93), with precipitation of magnetite in serpentine veins; and 3, alteration of serpentine into chlorite. We computed the reactions associated with alteration process for the above stages. In addition, thermodynamic modelling in multicomponent mineral equilibrium phase diagrams indicate that during the serpentinisation process, the desilication of pyroxene in caused the enhancement of SiO2 activity and restricted the transformation from olivine to magnetite. The reaction temperature for the serpentinisation of these rocks was relatively high, and within this temperature interval (100–300°C), lizardite was stable, and precipitation of magnetite was not favoured. We also evaluate the factors influencing magnetite precipitation during serpentinisation of ultramafic rocks including the composition of primitive rocks and the activity of SiO2 and temperature. Our study provides insights on the mechanism of magnetite mineralisation associated with the serpentinisation of ultramafic rocks.

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Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)

Cited by 43 publications

References 102 publications

Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater

Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater

Intra-oceanic arc growth driven by magmatic and tectonic processes recorded in the Neoproterozoic Bougmane arc complex (Anti-Atlas, Morocco)

Serpentinisation and magnetite formation in the Angwu ultramafic rocks from the central Bangong–Nujiang suture zone, Tibetan Plateau

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