Beryl occurrences in the granitic complex of Guerioune, Laouni, Southeastern Algeria: electron microprobe, infrared spectroscopy, and fluid inclusions data

Abstract

The Guerioune beryl is green or light green; the color is mainly due to the presence of Cr substituting Al in the beryl structure. The Cr2O3 and V2O3 contents in the Guerioune beryl are higher than in the other beryls of the Sahara region but lower than in the analyzed emeralds from Brazil (Capoierana, Nova Cruzeiro Ribeirao de Fogo). It remains unclear if the chromium in the beryl of Guerioune originated from the mafic rocks of the Laouni metamorphics or had a different origin. The Guerioune tardi-to post-cinematic massif intruded at shallow depth during the Pan-African orogeny (535–520 Ma) the Laouni terrane which is located 230km southwest of Tamanrasset, Algeria (Fig. 1). Guerioune granites are a peraluminous rare metal granites (RMG) poor in phosphorus but hosts beryl occurrences. The post-orogenic granites of Laouni Terrane form a set of small cupolas, aligned along with trending regional fault directed by NE-SW (Fig. 1b). The central part of the Guerioune massif is mainly composed of pink biotite granites that contains rounded enclaves of fine-grained granite. Stockscheider pegmatite type marks the contact to the albite–topaz granite. A muscovite granite and a porphyric biotite microgranite occurs as dike in the NE and NW of the massif. Beryl is found in three different associations: (i) pegmatites occur under quartz vein in muscovite granite (most of the beryl occurrences are associated with this type of pegmatite), (ii) in miarolitic cavities in the pink biotite–granite, and (iii) rarely in pegmatite veins. The micro-thermometric and Raman studies reveal a low salinity of the fluid inclusions (<10% NaCl equivalent) and a vapor phase consisting of three components (CO2 with low amounts of CH4 and N2). Solid phases identified in the fluid inclusions are secondary beryl, cristobalite, paragonite, and epistilbite. Temperatures show homogenization between 350° C and > 500° C. These results are in a total accordance with the hypothesis of magmatic or metamorphic fluids that are responsible of the crystallization of beryl.