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Characteristics of Hydrothermal Fluids Forming the Guelb Moghrein Fe Oxide-Cu-Au-Co Deposit, Mauritania: Ore Mineral Chemistry, Fluid Inclusions and Isotope Geochemistry
by
Jochen Kolb,  Department of Petrology and Economic Geology, Geological Survey of Denmark and Greenland, Copenhagen, Denmark,  Michael Meyer,  Stephan Prantl,  Sven Sindern,  Gregori A. Sakellaris,  Institute of Mineralogy and Economic Geology, RWTH Aachen University, Aachen, Germany,  Torsten Vennemann,  Institute of Mineralogy and Geochemistry, University of Lausanne L'Anthropole, Lausanne, Switzerland,  Michael E. Böttcher,  Leibniz Institute for Baltic Sea Research, Marine Geochemistry, Warnemünde, Germany.

in - Porter, T.M. (ed.), 2010 - Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, v. 4,  Advances
      in the Understanding of IOCG Deposits; PGC Publishing, Adelaide.  
pp. 553-572.

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ABSTRACT

The Guelb Moghrein iron oxide-Cu-Au-Co deposit is located about 250 km northeast of Nouakchott, the capital city of the Islamic Republic of Mauritania. It occurs on the western margin of the West African craton in an Archaean carbonate facies BIF, composed of meta-carbonate with pistomesite and magnetite, as well as Fe-Mg clinoamphibole-chlorite schist with additional magnetite, apatite, calcite, quartz, monazite and allanite. The Iron oxide-Cu-Au-Co mineralisation formed at 2492±9 Ma, during regional shearing in the upper greenschist/lower amphibolite facies, retrogressing rocks from amphibolite facies peak metamorphism.
   Host rocks to the hydrothermal mineralisation are breccias and shear zones which served as the principal pathways focusing the flow of ore fluids. Pebble-like breccia hosts the ore mineral assemblage comprising magnetite, pyrrhotite, chalcopyrite, Fe-Co-Ni arsenides, arsenopyrite, cobaltite and Bi-Au-Ag-Te minerals. A distal alteration developed in the wall rock amphibolite with the assemblage biotite, actinolite, grunerite and andalusite.
   The meta-carbonate hosts three types of aqueous fluid inclusions including Na, Mg, Ca and Fe with a Cl-molality of up to 6 mole/kg. Two types show characteristics of fluid mixing at 0.75 to 1.8 kbar and 410±30°C related to the hydrothermal mineralisation. The preferred interpretation of the H-, C-, O-, S- and Sr-isotope composition is that the original BIF signature was reset during the subsequent hydrothermal alteration by fluids from other crustal sources. The hydrothermal IOCG mineralisation is the result of a complex hydrothermal evolution involving crustal fluids focused in shear zones and mixed at the deposit site. Gold was transported as an AuCl
2-complex in the undersaturated ore fluids and concentrated in coexisting Bi melt-droplets resulting in the precipitation of an Au-Bi-assemblage during cooling.

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