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Degassing as the Main Ore-Forming Process at the Giant Imiter Ag–Hg Vein Deposit in the Anti-Atlas Mountains, Morocco

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Mineral Deposits of North Africa

Part of the book series: Mineral Resource Reviews ((MIRERE))

Abstract

The giant Imiter epithermal Ag–Hg vein deposit in the Anti-Atlas Mountains of southern Morocco formed during a major episode of mineralization linked with Ediacaran volcanism at ca. 550 Ma. Silver was deposited during two main epithermal mineralizing events referred to as epithermal-quartz (ESE-Qz) and epithermal-dolomite (ESE-Dol) stages under distinct stress fields (i.e., WNW-ESE and N-S shortening directions), and is confined to the late Neoproterozoic, N60-90° E-trending, transcrustal Imiter fault zone. Economic orebodies are aligned mainly along the interface between sedimentary and volcanic units of lower and upper Cryogenian age. The ore mineralogy consists principally of Ag–Hg amalgam, argentite, polybasite, pearceite, tetrahedrite-tennantite, proustite-pyrargyrite, imiterite, acanthite, arsenopyrite, pyrite, sphalerite, and galena. Gangue constituents are dominated by quartz (ESE-Qz stage) and dolomite (ESE-Dol stage). Wall-rock alteration is well developed and includes silicification and dolomitization, and minor propylitization and kaolinitization. Fluid inclusion data indicate that the mineralizing fluids evolved through time, from a mean temperature of ~180 °C and salinity of ~10 wt% NaCl during ESE-Qz stage I, to a mean temperature of ~165 °C and salinity of ~24 wt% NaCl equiv during ESE-Dol stage II. Calculated trapping pressures, in the range of 1.1–0.9 kbar, exclude fluid unmixing “effervescence” as a viable ore depositional mechanism. Conversely, halogen compositions suggest the involvement of magmatic brines and evolved seawater. Stable (C, O, S) and radiogenic (Pb, Re/Os) isotope data, together with noble gas isotope compositions, are consistent with various degrees of mixing between mantle and crustal sources along the fluid flow path. Collectively, these data suggest that degassing of CO2 and SO2 during epithermal mineralization and related fluid/rock interactions led to local redox-potential decreases and pH increases that resulted in preferential deposition of massive amounts of native Hg-rich silver instead of Ag and Hg sulphide minerals.

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Acknowledgments

Special thanks to Marina Vega and Teresa Soledad Medina Malagón for assistance at the CGEO laboratory. This study was financed by UNAM-PAPIIT project IN110912 and CONACYT projects 81584 and 80142. We also thank John Slack for his constructive review, which led to numerous improvements in the quality and clarity of the text.

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Authors and Affiliations

  1. Programa de Geofluidos, Centro de Geociencias UNAM-Campus Juriquilla, AP 1-253, CP 76230, Juriquilla, Querétaro, Mexico

    Gilles Levresse & Jordi Tritlla

  2. Laboratoire des Gîtes Minéraux, Hydrogéologie & Environnement, Faculté Des Sciences, Université Mohammed Premier, 60000, Oujda, Morocco

    Mohammed Bouabdellah

  3. ENSG, Géoressources, Université de Lorraine, 2 rue du Doyen Marcel Roubault, 54518, Vandoeuvre-les-Nancy, France

    Alain Cheilletz

  4. Université de Savoie Mont Blanc, SceM-EDYTEM, CNRS-UMR 5204, Campus Scientifique, 73376, Le Bourget du Lac Cedex, France

    Dominique Gasquet

  5. Managem Group, Twin Center, Tour A, Angle Boulevards Zerktouni et al Massira al Khadra, B.P. 5199, 20000, Casablanca, Morocco

    Lhou Maacha

  6. Repsol Exploración, Paseo de la Castellana 280, 28046, Madrid, Spain

    Jordi Tritlla

  7. School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, UK

    David Banks

  8. AXMINE Company, Yougoslavie Avenue, Immeuble106/6, Guéliz, 40000, Marrakech, Morocco

    Azizi Samir Moulay Rachid

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  1. Earth Sciences, Mohammed Ist University, Oujda, Morocco

    Mohammed Bouabdellah

  2. U.S. Geological Survey (Emeritus), Farmington, Maine, USA

    John F. Slack

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Levresse, G. et al. (2016). Degassing as the Main Ore-Forming Process at the Giant Imiter Ag–Hg Vein Deposit in the Anti-Atlas Mountains, Morocco. In: Bouabdellah, M., Slack, J. (eds) Mineral Deposits of North Africa. Mineral Resource Reviews. Springer, Cham. https://doi.org/10.1007/978-3-319-31733-5_2

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