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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


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 T h 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 SO4 2− 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.

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We are grateful to Cindy Kestler, Wayne Premo, and Craig Johnson of the USGS in Denver for stable and radiogenic isotope analyses. D. Banks (School of Earth Sciences of the University of Leeds) and Z. Sharp (Department of Earth and Planetary Sciences, University of New Mexico, USA) are thanked for prompt and reliable crush leach analyses and δD data of the samples presented in this paper. We are also indebted to D. Sangster for his helpful comments on earlier versions of the manuscript. We thank the two Mineralium Deposita reviewers D. Banks and C. Marignac for constructive critical reviews and input that substantially improved the manuscript. C. Marignac provided invaluable assistance and insightful comments on an earlier version of this manuscript. Special thanks to B. Lehmann, the Chief Editor of Mineralium Deposita, for his patience, thorough reviews, and helpful comments.

Funding for this research was provided through grants from the Programme d’Appui à la Recherche Scientifique of Morocco (PROTARS II/ P23/33), the Moroccan–Spanish Scientific Research Program (188/04/RE), and the NATO Fellowship (EST.CLG.979371) and was supported by a Fulbright postdoctoral fellowship awarded to the first author.

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Correspondence to Mohammed Bouabdellah.

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Editorial handling: B. Lehmann.

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Bouabdellah, M., Beaudoin, G., Leach, D.L. et al. 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. Miner Deposita 44, 689 (2009). https://doi.org/10.1007/s00126-009-0232-8

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