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).

Benaouda, Rachid, Devey, Colin W. , Badra, Lakhlifi and Ennaciri, Aomar (2017) 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). Journal of Geochemical Exploration, 177 . pp. 28-44. DOI 10.1016/j.gexplo.2017.02.002.

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Abstract

Highlights

• The Jbel Boho alkaline igneous complex is characterized by LREE ore-bearing and ore-barren quartz-carbonate veins.
• The mineralized veins occur around syenite showing synchysite-(Ce) as main LREE-host mineral.
• Fluid inclusion study suggests the transport of LREE by highly saline fluids as Cl-complex under acidic pH conditions.
• LREE mineralization results from LREE-bearing fluids neutralization with veins carbonate or by mixing with meteoric fluids.

The world's largest mined rare-earth element deposits are associatedwith alkalinemagmatism,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 veinswhich contain
at least twosilica generations. Only the second quartz generation is associatedwith LREE mineralization. The predominant
REE-host mineral is the Ca-LREE-fluorcarbonate synchysite-(Ce), mainly present as anhedral crystals
up to 200 μm. Somevery small anhedral rhabdophane-(Ce) grainswere also found in a synchysite-bearing quartz
vein. Thermometric studies on mainly liquid-vapour-solid fluid inclusions in quartz crystals in the synchysitebearing
veins suggest very high salinity (32 to 37 wt% 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 lowpH 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 CO3 2– 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.

Document Type: Article
Additional Information: This study is part of RB's PhD thesis at the University of Kiel - WOS:000399848000003
Keywords: Synchysite, Rhabdophane, Hydrothermal, Quartz veins, REE mineralization, Jbel Boho, Rare-earth elements, Morocco, Cambrian, alkaline, syenite
Research affiliation: OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-MUHS Magmatic and Hydrothermal Systems
Kiel University
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1016/j.gexplo.2017.02.002
ISSN: 0375-6742
Date Deposited: 06 Mar 2017 10:58
Last Modified: 01 Feb 2019 15:13
URI: http://oceanrep.geomar.de/id/eprint/36808

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