Petrology, geochemistry, stable and radiogenetic isotopy of the Guelb Moghrein iron oxide copper gold cobalt deposit, Mauritania

The Guelb Moghrein Fe oxide-Cu-Au-Co deposit is located in the central Mauritanides fold and- thrust belt, in the Akjoujt region, northern Mauritania. The deposit is hosted by an Archean metavolcanosedimentary sequence at the western boundaries of the West African Craton. Sericite-quartz- and biotite garnet-quartz schists that belong to the Saint Barbe Volcanic unit make up the stratigraphic base of the lithological sequence in the pit. The sericite-quartz schist represents a metatuff of rhyodacite to rhyolite composition and the biotite-garnet-quartz schist is an intercalating metapelite. A suite of metavolcanic rocks of the Akjoujt Metabasalt unit including massive amphibolite lenses, biotite-actinolite- and chlorite schists dominate the deposit area. Textural and geochemical analyses indicate that the precursor volcanic rocks are submarine, andesites/basalts crystallized in a volcanic arc/continental margin setting. The Fe oxide-Cu-Au-Co mineralization is hosted by a lensoid metacarbonate body in the central part of the pit area. The metacarbonate is a massive, very coarse-grained rock, consisting of up to 5 cm large siderite crystals. A line of evidence including geochemical and stable isotopic signature of the siderite indicates that the metacarbonate represents marine metasediment deposited in a continental shelf setting. Thermobarometric analyses on the regional lithologies record a clockwise retrograde P-T path subsequent to thrust deformation events. Peak, amphibolite facies metamorphism for the amphibolite of the Akjoujt Metabasalt unit was calculated at 580 ±40°C and 5 kbar. Retrograde shearing of the amphibolite during the northeastward D2 regional thrust event resulted in the development of the S2 foliation and the formation of the biotite-actinolite schist. This retrograde overprint occurred at 410 ±30°C and ca. 3 kbar. The D2 event resulted in overthrusting of the Akjoujt Metabasalt unit by the Saint Barbe Volcanic unit as well as the IOCG hydrothermal mineralization in the metacarbonate. Subsequent eastward D3 thrusting resulted in the retrogression of amphibolite to chlorite schist under retrograde lower greenschist facies metamorphism. The shape analysis of the metacarbonate body indicates that its lensoid geometry is the result of planar alignment and thin-stacking during the D2 event. The economic Fe oxide-Cu-Au-Co mineralization at Guelb Moghrein is confined to discrete D2 shear zones that truncate the metacarbonate body. The ore bodies occur in breccia zones developed in the metacarbonate and form multiple, up to 30 m wide, tabular, foliation (S2) parallel lenses that dip moderately SW. The breccia zones have a characteristic geometry of central, ductile Fe-Mg clinoamphibole-chlorite phyllonite surrounded by a progressively developed breccia of fractured siderite. In these damage zones, a massive sulfide-arsenide-gold assemblage formed together with low-Ti magnetite, Fe-Mg clinoamphiboles, and graphite in more distal parts replacing the brecciated siderite. Geochemically, the breccia zones are characterized by substantial enrichment of several elements that include Fe, Cu, Au, and Co as well as Si, S, As, U, Th, F, REE, Bi, Ag, Bi, and Te. Peripheral to the metacarbonate in the Akjoujt Metabasalt unit a ca. 40 m wide biotitechlorite- grunerite-calcite alteration halo developed during hydrothermal alteration. The bulk of the alteration assemblage occurs in the recrystallized feldspar matrix; however this zone is not significantly mineralized. Towards the ore bodies, the biotite-chlorite-grunerite-calcite alteration halo records progressive enrichment in K, Fe, and Mg, REE, S and Cu and depletion in Na, and Sr relative to least altered amphibolite. The primary ore mineralogy in the breccia zones is dominated by pyrrhotite and chalcopyrite and includes Fe-Co-Ni arsenides, arsenopyrite, cobaltite, Bi-Au-Ag-Te minerals as well as magnetite, Fe-Mg clinoamphiboles, chlorite, graphite, apatite, xenotime and monazite. The paragenetic assemblages indicate a relative high formation temperature (400-450°C) in agreement to the calculated retrograde metamorphic conditions. The principal metal, gold, is found either in native form, in solid solution with silver (electrum) or bismuth (maldonite) or in a complex Bi-Au-Ag-tellurides and is associated with arsenopyrite and clinosafflorite. Exsolution mineralogy includes troilite, pentlandite, cubanite, mackinawite and nickeline. This assemblage was formed in a temperature range of 200-250°C, under lower greenschist facies metamorphism, during D3. The isotopic composition of siderite from the breccia is characterized by depletion of d13C values and enrichment of d18O values attributed to the partial decarbonation of siderite and subsequent equilibration with the hydrothermal fluid. The occurrence of graphite with siderite and magnetite, and the very light isotopic composition of graphite are consistent with an abiogenic origin of graphite by carbonate reduction of siderite at temperatures ca. 430°C. The isotopic composition of the hydrothermal fluid responsible for the mineralization at the Guelb Moghrein was calculated from the measured d18O, dD and d34S values of paragenetic minerals from the breccia zones. The calculated isotope values obtained are consistent with a single fluid of metamorphic origin. Radiometric U-Pb and Pb-Pb dating applied on hydrothermal REE-phosphates from the main ore zones addresses the hydrothermal activity at Guelb Moghrein and consequently the deformational events on regional scale. Phosphates associated with the primary sulfide ore assemblage are type I monazite and xenotime. The age obtained for type I phosphates of 2492 ±9 Ma is interpreted to reflect the age of hydrothermal fluid flow and associated ore formation during regional D2 deformation. Type II monazite and xenotime as well as the composite type III grains are also hydrothermal in origin, but they are not part of the primary ore mineral assemblage. They formed because of localized fluid flow and fluid–rock reaction during subsequent D3 thrusting. Textural and chemical evidence indicate that type I phosphates that were exposed in the later D3 shear zones to hydrothermal fluid flow reacted with the fluid to form either new monazite and, to a much lesser extent, xenotime grains with complete isotopic resetting or, in cases of incomplete dissolution, secondary phosphate overgrowths formed on older cores, as indicted by the composite grains. Timing of the D3 event and fluid flow is recorded by the age of type II phosphates of 1742 ±12 Ma