Genesis of the Assif El Mal Zn-Pb (Cu, Ag) vein deposit. An extension-related Mesozoic vein system in the High Atlas of Morocco. Structural, mineralogical, and geochemical evidence

Mohammed Bouabdellah, Georges Beaudoin, David L. Leach, Fidel Grandia, Esteve Cardellach

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16 Citations (Scopus)

Abstract

The Assif El Mal Zn-Pb (Cu-Ag) vein system, located in the northern flank of the High Atlas of Marrakech (Morocco), is hosted in a Cambro-Ordovician volcaniclastic and metasedimentary sequence composed of graywacke, siltstone, pelite, and shale interlayered with minor tuff and mudstone. Intrusion of synorogenic to postorogenic Late Hercynian peraluminous granitoids has contact metamorphosed the host rocks giving rise to a metamorphic assemblage of quartz, plagioclase, biotite, muscovite, chlorite, amphibole, chloritoid, and garnet. The Assif El Mal Zn-Pb (Cu-Ag) mineralization forms subvertical veins with ribbon, fault breccia, cockade, comb, and crack and seal textures. Two-phase liquid-vapor fluid inclusions that were trapped during several stages occur in quartz and sphalerite. Primary inclusion fluids exhibit Th mean values ranging from 104°C to 198°C. Final ice-melting temperatures range from -8.1°C to -12.8°C, corresponding to salinities of ∼15 wt.% NaCl equiv. Halogen data suggest that the salinity of the ore fluids was largely due to evaporation of seawater. Late secondary fluid inclusions have either Ca-rich, saline (26 wt.% NaCl equiv.), or very dilute (3.5 wt.% NaCl equiv.) compositions and homogenization temperatures ranging from 75°C to 150°C. The δ18O and δD fluid values suggest an isotopically heterogeneous fluid source involving mixing between connate seawater and black-shale-derived organic waters. Low δ13CVPDB values ranging from -7.5‰ to -7.7‰ indicate a homogeneous carbon source, possibly organic matter disseminated in black shale hosting the Zn-Pb (Cu-Ag) veins. The calculated δ34SH2S values for reduced sulfur (22.5‰ to 24.3‰) are most likely from reduction of SO42- in trapped seawater sulfate or evaporite in the host rocks. Reduction of sulfate probably occurred through thermochemical sulfate reduction in which organic matter was oxidized to produce CO2 which ultimately led to precipitation of saddle dolomite with isotopically light carbon. Lead isotope compositions are consistent with fluid-rock interaction that leached metals from the immediate Cambro-Ordovician volcaniclastic and metasedimentary sequence or from the underlying Paleo-Neoproterozoic crustal basement. Geological constraints suggest that the vein system of Assif El Mal formed during the Jurassic opening of the central Atlantic Ocean. © Springer-Verlag 2009.
Original languageEnglish
Pages (from-to)689-704
JournalMineralium Deposita
Volume44
Issue number6
DOIs
Publication statusPublished - 24 Feb 2009

Keywords

  • Assif El Mal
  • Fluid inclusions
  • Morocco
  • Stable isotopes
  • Zn-Pb vein deposits

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