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| Titel |
Massive magnetite veins in Neoproterozoic serpentinites as a possible relic of a black smoker type hydrothermal system: Aït Ahmane ultramafic unit (Bou Azzer ophiolite, Anti Atlas, Morocco) |
| VerfasserIn |
Florent Hodel, Mélina Macouin, Antoine Triantafyllou, Julien Berger, Julie Carlut, Nasser Ennih, Sonia Rousse, Ricardo Trindade |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250143861
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| Publikation (Nr.) |
 EGU/EGU2017-7624.pdf |
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| Zusammenfassung |
| Ophiolites are the only remains of ancient oceanic lithosphere (> 200 Ma), and
of the processes affecting it like serpentinization and hydrothermal alteration. If
magnetite is a common serpentinization product, centimetric, massive and almost
pure magnetite veins are rarely observed in serpentinites. Unique examples of such
veins, in the Aït Ahmane ultramafic unit (Bou Azzer Neoproterozoic ophiolite,
Anti-Atlas, Morocco), offer the opportunity to assess the hydrothermal processes
that prevailed at the end of the Precambrian. Here, magnetic data, petrographic
observations, mineral and bulk chemistry are combined to assess iron behavior in these
meta-ultramafics and constrain the serpentinites alteration and magnetite veins formation
processes.
Very high Cr# and low Mg# of Cr-spinel cores in serpentinites together with low bulk rock
incompatible major element concentrations (Al2O3: 0.20-1.28 wt. % and Ti: 3-38 ppm)
and very low REE support a highly refractory protolith, characteristic of a supra
subduction context. Typical lizardite/chrysotile pseudomorphic texture in fresh
serpentinites reveals an initial oceanic-like serpentinization, involving temperature
<350 ∘C while the abundance of magnetite (up to 7.86 wt. %) in these unaltered
serpentinites attests of a relatively high serpentinization temperature >200 ∘C. Magnetic
measurements reveal a lower magnetite content in hydrothermalized serpentinites hosting the
magnetite veins, with lowest values (down to 0.58 wt. %) for bleached serpentinites
constituting the wall rock of veins. These magnetic data are consistent with bulk rock
chemistry that shows a lower total iron content in hydrothermalized serpentinites
hosting the veins. Hydrothermalized samples exhibit a strong LREE enrichment
(La/Yb to 212.66), correlated to a positive peak for Eu ((Eu/Eu*)N up to 27.43). This
hydrothermal imprint is clearly anticorrelated with total iron content, attesting the
important iron mobilization during this fluid/rock interaction. Mineral chemistry
reveals a significant chlorine enrichment in serpentine phases from hydrothermalized
serpentinites.
In Aït Ahmane, a Cl-rich acidic hydrothermal event involving temperatures below 350 ∘C
appears to have been responsible of an intense magnetite leaching in host serpentinite. Iron
provided by this leaching may have conducted to unique magnetite veins formation in the Aït
Ahmane ultramafic unit. Two different settings are proposed for the nature of the
hydrothermal event: (1) a continental hydrothermal system as advanced for the Co-Ni-As ores
in the Bou Azzer inliers or (2) an oceanic black smoker type hydrothermal vent field on the
Neoproterozoic sea-floor. |
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