The Eugui-Asturreta magnesite deposit (Western Pyrenees, Spain) forms a discoidal body with a maximum thickness of 130 m, located within a folded Namurian carbonate sequence. The magnesite rocks are composed of lens-shaped crystals (max 8 cm) arranged in black and white bands (zebra structure). The morphology and textural characteristics of magnesite and the structural relationships between magnesite and dolostone host rocks indicate that magnesite replaced the host dolostone.

The magnesite crystals replaced the host dolostones, growing perpendicularly from stylolites and meeting between two adjacent stylolite sets. The portions of the crystals close to the stylolites are black colored because they incorporated carbonaceous matter and clay laminations of the replaced dolostone (incorporative growth), whereas the crystal terminations are white because impurities have been displaced and lie at an intercrystalline position and into the meeting zone (displacive growth). These texture differentiations could be interpreted by changes in the ratios of dolomite dissolution versus magnesite precipitation due to increasing amounts of Ca released during the replacement process. Late dolomite is commonly pseudomorphous after magnesite crystals.

Dolostone host rocks and magnesite show similar Mn, Al, and K contents. The host dolostones have a lower average FeO content (0.24 wt %) than magnesite (1.82 wt %) and than later dolomite after magnesite (1.04 wt %). The black magnesite bands with a low percentage of impurities show REE contents and patterns similar to those of the relatively pure dolostone host rocks, suggesting an origin by metasomatic replacement. The white magnesite bands display REE contents similar, but slightly lower in LREE, to those of the host dolostones, as would be expected by magnesite replacing dolomite. REE patterns of dolomite after magnesite are very similar to those of magnesite, suggesting that REE behaved conservatively during the replacment of magnesite by dolomite and that the process has been induced by fluids impoverished in REE and equilibrated with the carbonate sequence.

The petrography and geochemistry of the Eugui magnesite indicate that the deposit originated by metasomatic replacement of a dolostone precursor promoted by fluids moving through stylolite and bedding planes. Sedimentary, structural, textural, and geochemical relics of the replaced dolostones are preserved in the magnesite rocks. The peculiar textural characteristics of the magnesite represent replacement features and cannot be interpreted by diagenetic or metamorphic recrystallization of an original marine microcrystalline magnesite deposit.

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