Light rare-earth element mineralization in hydrothermal veins related to the Jbel Boho alkaline igneous complex, AntiAtlas/Morocco: The role of fluid-carbonate interactions in the deposition of synchysite-(Ce)

Abstract

The world's largest mined rare-earth element deposits are associated with alkaline magmatism, making it important to understand the mechanisms leading to magmatic and hydrothermal element enrichment. We present results from late-differentiation-stage hydrothermal veins of the Jbel Boho alkaline complex in the district of Bou Azzer in the Anti-Atlas of Morocco, which show high light-REE enrichments. The REE mineralized veins occur around a syenitic pluton at the centre of the complex and consist of quartz-jasper and quartz veins which contain at least two silica generations. Only the second quartz generation is associated with LREE mineralization. The predominant REE-host mineral is the Ca-LREE-fluorcarbonate synchysite-(Ce), mainly present as anhedral crystals up to 200μm. Some very small anhedral rhabdophane-(Ce) grains were also found in a synchysite-bearing quartz vein. Thermometric studies on mainly liquid-vapour-solid fluid inclusions in quartz crystals in the synchysite-bearing veins suggest very high salinity (32 to 37wt% NaCl equiv.) of the mineralizing fluid. Homogenization temperatures from 150° to 250°C provide the minimum temperature conditions in which the quartz veins were formed.Based on recent experimental data, we propose a model for the formation of hydrothermal REE-Ca-F carbonate deposits in the veins, involving the transport of REE as chloride complexes at low pH conditions in the presence of fluoride ions. The deposition of synchysite is proposed to result from neutralization of this fluid by mixing of hydrothermal ore fluids with carbonate-rich meteoric water or by interaction with already existing carbonates in the vein, As well as raising the pH, this interaction also provides the Ca+ and CO32– ions needed for REE precipitation as Ca-F carbonates.The barren veins show two types of mineralogy and REE patterns: quartz-carbonate veins with enrichment of LREE over HREE and iron-rich jasper-bearing veins with quite flat REE pattern and high HREE. The LREE/HREE fractionation in these veins seems to be controlled by an interplay of two factors: 1) a low activity of ligands like Cl, which favours LREE transport and (2) the mineralogical control, by which HREE having similar ionic radii to Fe2+ will be preferred over LREE.