Granite-Hosted Fluorite–Barite Veins at Jbel Merguechoum (Northeastern Morocco): Paragenesis, Trace-Elements and REE Geochemistry, and Constraints on the Origin of Green and Purple Coloration

The Jbel Merguechoum fluorite–barite deposit in northeastern Morocco is a granite-hosted fluorite–barite vein-type deposit whose trace and rare-earth element (REE) geochemistry, role of structural defects and trace elements in fluorite coloration, and specimen quality of the fluorite in relation to geoheritage are still poorly constrained. The site has never been drilled or mined on a commercial scale, and no reserve estimate exists. This study addresses these gaps through field and hand-specimen observations and ICP-MS analyses of green and purple fluorite samples and host Neovariscan granite. The mineralization occurs inside the granite as disseminated replacements, tension-gash fillings, geodes, and open-space vein fills, distributed along NNE-SSW to ENE-WSW normal faults. Crystal habit varies systematically with depth, and this distribution yields a range of well-preserved, museum-grade habits that are characterized here for the first time. Fluorite samples are strongly depleted in high-field-strength elements (Zr, Nb, Hf, Ta ≤ 0.33 ppm) and in thorium and uranium (Th 0.18–0.31 ppm; U 0.19–0.32 ppm), which is one to two orders of magnitude below the values measured in the granite. Among the large-ion lithophile elements, only barium is high (712–766 ppm), most probably because of barite micro-inclusions, whereas cesium (0.13–0.26 ppm) and rubidium (0.21–0.34 ppm) are very low. Total REE contents of the fluorite (17.2–19.7 ppm) are roughly one order of magnitude lower than those of the granite (189 ppm). Y/Ho ratios of 110 in the green cube’s fluorite and 135 in the purple tetrahedra fluorite are strongly superchondritic and contrast with the chondritic ratio of 28 obtained for the granite. These signatures are more consistent with precipitation from fluids that circulated through and leached the granite than from a magmatic fluid exsolved directly from it. A mass balance shows that barite micro-inclusions (≤0.13 wt%) can explain most of the Ba and Sr of the fluorite separates but have a negligible effect on the REE, Y/Ho and Eu/Eu* values. These geochemical signatures indicate precipitation from fluids that had circulated through and leached the granite, rather than from a magmatic fluid exsolved directly from it. The very low uranium and thorium contents of the purple fluorite make internal self-irradiation an unlikely dominant cause of its colour. However, they do not identify the responsible chromophore. Similarly, the presence of samarium in the green variety does not prove that samarium is the chromophore, as bulk Sm contents of the two colour types overlap. Colour models are therefore put forward as working hypotheses that still must be tested spectroscopically. Jbel Merguechoum holds particular significance for both high-level mineralogical research and collectors.