A COMBINED GENESIS FOR EMERALD FORMATION: A FIELD AND ANALYTICAL STUDY OF THE EMERALDS DEPOSIT OF IRONDRO, MANANJARY AREA, MADAGASCAR

Emerald, a highly desirable and valuable gemstone, is a Beryllium-rich silicate with the general formula: Be₃Al₂(Si₆O₁₈) that owes its green color and rarity to trace amounts of Cr, V and/or Fe. Today there are only 41 known emerald deposits distributed in 20 different countries genetically classified into 3 main types, magmatic, metamorphic and sedimentary with metasomatic fluids involve in all of them. However, recent research point towards a combination of igneous and metamorphic processes as a possible classification scheme. Moreover, plausible tectonic environments of emerald formation still not well understood. The aim of this study is to refine the genetic classification scheme and more importantly understand the relationship between emerald formation and their associated sources through the study of the emeralds deposit of Irondro, Mananjary region, Eastern Madagascar.

Emeralds from the Irondro deposit are mainly concentrated along black-wall reaction zones (amphibolite-phlogopite-rich rocks occasionally pillowed) between migmatitic gneiss, talc-schist and lenses of chromite-bearing serpentinites. Lack of exposure, intense vegetation and highly metamorphosed terrains lead to difficult field observations and interpretation of the structural relationships between the different lithological units. Here we present preliminary analytical results, whole rock and trace element studies, in conjunction with field observations that indicate fluid/rock migration/interaction during regional metamorphism as their genesis of formation. Moreover, a peraluminous fluid seems to exert a strong control in emerald formation.

Emeralds mirror the geochemical signature of their associated sources: host rock and magma source. In conjunction with interpreting and understanding these relationships alongside the tectonic environments of formation and emplacement, we may be able to further aid the exploration and economical viability of deposits using a systematic/scientific approach.