Metacarbonatite rocks from Amesmessa area (In Ouzzal Terrane), Hoggar shield, Algeria

The Hoggar Shield (South Algeria) belongs to the 3,000 km-long Pan-African Trans-Saharan Belt which formed in the Neoproterozoic and Cambrian, between 750 and 545 Ma. At the western part of this shield, carbonate rocks crop out to the north and south of Amesmessa gold deposit. They occur as lensoid bodies and dikes cross-cutting Archean granulitic rocks, and are consistently associated with mafic rocks and syenite. The petrogenesis of the Amesmessa metacarbonate rocks, which have previously been interpreted as marbles of Archean age, are reconsidered in the present study. Within the Amesmessa area, a small volume of carbonate rocks consisting of calcite, biotite, clinopyroxene, hornblende, olivine and apatite, and are spatially associated with ultramafic rocks, are surrounded by granosyenite, pyroxenite and rhyolite. The cross-cutting relationship and other outcrop features of the area indicate that the carbonate rocks are an intrusive phase and younger than the other surrounding Archean rocks. The average total REE concentration in the metacarbonate is up to 5,700 ppm. However the associated rocks; syenite and pyroxenite contain only 60 and 12 ppm, respectively. Sr concentration is also high (>1,390 ppm) which is normally high in carbonatitic rocks and low in limestones. On the basis of available chemical analyses and outcrop evidence, it becomes clear that the Amesmessa carbonate rocks consist of metamorphosed carbonatite.