Abstract

The Guelb Moghrein Cu-Au deposit occurs within metacarbonate bodies derived from sedimentary carbonate beds of probable late Archean age. The metacarbonate lenses are enclosed within mafic meta-igneous rocks that display a coarse-grained, amphibolite-facies (hornblende-albite) assemblage that is mineralogically anomalous relative to the greenschist-facies metamorphic rocks present regionally within the district and may represent a zone of synmetamorphic hydrothermal metasomatism. Crosscutting relationships and textural evidence for progressive replacement of different mineral assemblages demonstrate that the deposit had a complex, multiphase alteration-mineralization history. Early sodic alteration at Guelb Moghrein resulted in the formation of albitized zones along moderately to shallowly dipping structures. Sodic alteration was followed by syndeformational ferroan potassic alteration that resulted in the growth of biotite and grunerite-cummingtonite in meta-igneous rocks and conversion of calcite-dolomite in metacarbonate bodies to ankerite- and siderite-rich assemblages. Progressive alteration resulted in breakdown of siderite to magnetite and graphite at approximately 400° to 450°C. Sulfide mineralization took place at ca. 2.5 Ga, concurrently with or shortly after ferroan potassic alteration and magnetite-graphite formation. An early assemblage of Co-, Ni-, and As-rich sulfide minerals was followed by precipitation of pyrrhotite and chalcopyrite. Sulfide mineralization was focused within brittle deformation zones along the margins of the metacarbonate bodies, which represent high-permeability zones adjacent to more ductile structures. The nucleation of sulfides on graphite during siderite breakdown suggests redox reactions may have largely controlled sulfide precipitation. A late magnesian alteration event at ca. 1.74 Ga resulted in the formation of chlorite-rich shear zones and precipitation and/or recrystallization of sulfides. The Guelb Moghrein deposit is a member of the iron oxide Cu-Au class of deposits but is unusual in being hosted in carbonate strata.

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