REE and HFSE mineralization in peralkaline granites of the Ambohimirahavavy alkaline complex, Ampasindava peninsula, Madagascar

• Rare metals are hosted in granitic dykes emplaced in the rims of the complex.
• Granites are ferroan and have a peralkaline affinity.
• Mineralization is agpaitic and miaskitic magmatic, and hydrothermal.
• The hydrothermal mineralization consists of pseudomorphs after primary minerals.
• These granites represent one of very few known occurrence of eudialyte in granite.

The Cenozoic Ambohimirahavavy alkaline complex in the northwest of Madagascar is characterized, in its outer flanks, by a network of discontinuous granitic dykes mineralized in rare-earth elements (REE) and high-field-strength elements (HFSE). These granitic dykes contain alkali amphibole and pyroxene, are strongly ferrian, have a peralkaline affinity [molar (Na2O + K2O)/Al2O3 > 1], but differ in textures and mineral assemblages. Based on the latter parameters, three varieties of granite could be distinguished: coarse-grained, pegmatitic and eudialyte-bearing granite. Detailed study of the REE- and HFSE-bearing minerals revealed the presence of both magmatic and hydrothermal phases. In coarse-grained granite (GR-I) prevails a magmatic miaskitic assemblage, rich in zircon. In the pegmatitic granite (GR-II), the rare metals are contained in secondary minerals, essentially amoeboid zircon and rare-earth fluorocarbonates, forming pseudomorphs after eudialyte. The eudialyte-bearing granite (GR-III), the richest in REE and HFSE, is characterized by a magmatic agpaitic assemblage, dominated by eudialyte. A skarn develops at the contact of GR-I with limestone. Mineralization in this zone is in the form of pseudomorphs of rare-metal-bearing minerals after aegirine–augite.Presence of a magmatic miaskitic vs. an agpaitic assemblage in the coarse-grained and eudialyte-bearing granites, respectively, is interpreted to result from crystallization of differently evolved melts that probably originated from a common basaltic parental magma. Origin from the melting of a metasomatized lower crust cannot be excluded. In the eudialyte-bearing granite, very high contents of Na and volatiles maintained the rare metals in the melt to the very last stages of crystallization, whereas in the coarse-grained granite, Na and volatiles were probably lost to an exsolving fluid, causing the precipitation of a miaskitic assemblage. Subsolidus remobilization of REE and HFSE took place via an alkali-, silica- and F-rich orthomagmatic fluid exsolved from the crystallizing peralkaline granitic melt.