Occurrence of high-grade LREE allanite-pegmatites and calcite carbonatite dykes in the Ediacaran complex of Aghracha, Oulad Dlim massif (South Morocco)

Abstract

High-grade REE-bearing granitic pegmatites and carbonatite dykes were recently discovered in the Aghracha area, South Morocco. They intruded the northern part of the Ediacaran complex, mainly composed of gneiss. The geochemistry of the basement samples displays compositions ranging from monzonitic to alkaline and sub-alkaline granites with prevailing positive Eu anomalies in chondrite-normalized patterns. These have been attributed to a magma mixing likely derived from the re-melting of underplated mafic rocks and a small amount of crustal rocks. The Aghracha pegmatites are rich in epidote and occur in the Awhifrit prospect in the western part of the basement. The analyzed samples display REE contents up to 17 wt%, mainly related to allanite-(Ce) (up to 13 cm in size), minor monazite-(Ce), and thorianite. The Aghracha pegmatite is referred to here as “allanite-pegmatite” because it exceeds the compositional fields of the classical LCT- or NYF-type classification. Calcite carbonatite dykes occur in the Laachariyat area in the western part of the Ediacaran complex. In addition to calcite, they also contain abundant strontianite, magnetite, barite, and apatite. The main REE-host mineral is monazite-(Ce), although minor bastnaesite and allanite also occur. Both pegmatites and carbonatite dykes strike in an NNW direction, likely associated with an extensional tectonic setting which is typical for the REE-bearing deposits and the Ediacaran complex of the Oulad Dlim massif.