Summary
The cobalt-arsenide ores of Bou Azzer are located along the borders of serpentinite massifs (Upper Proterozoic ophiolite complex) in carbonate-quartz lenses resulting from hydrothermal carbonate alteration of serpentinite. The cobalt ores contain an average gold content of 5–20 ppm; gold is mainly located in skutterudite (120 ppm av.), whereas the Fe-arsenide (loellingite) contains < 1 ppm Au. Similarly the highest PGE contents are found in skutterudite (up to 2 ppm total PGE). All the arsenide ores of Bou Azzer exhibit the same chondrite normalized PGE pattern displaying positive Rh and negative Pt anomalies, and a slight positive slope (Pd/Ir = 1 to 2). This uncommon PGE pattern closely resembles to that of sulphides of komatiites.
In serpentinite, the PGE patterns are typical of slightly depleted mantle rocks, and the associated podiform chromitites are within the range of ophiolitic chromitites, except for Pd and Au enrichment.
Horizons of sulphide-bearing serpentinites show relatively high contents of noble metals and display PGE patterns which closely resemble those of the Co-arsenide ores, although an order of magnitude lower. These sulphides probably correspond to the remobilization during serpentinization of primary magmatic sulphides. The sulphiderich horizons are a possible source-rock for the noble metals of the Bou Azzer cobaltarsenide ores.
Zusammenfassung
Die Kobalt-Arsenid Erze von Bou Azzer kommen entlang den Grenzen eines Serpentinit-Massifs (Oberproterozoischer Ophiolit-Komplex) in Karbonat-Quarz-Linsen vor, die auf hydrothermale Umwandlung des Serpentinits zurückgehen.
Die Kobalt-Erze enthalten 5–20 ppm Gold; dieses kommt hauptsächlich in Skutterudit (120 ppm) vor, während die Fe-Arsenide (Loellingit) weniger als 1 ppm Gold enthalten. Die höchsten PGE Gehalte kommen ebenso in Skutterudit vor (bis zu 2 ppm PGE). Alle Arsenid-Erze zeigen das gleiche Verteilungsbild mit positiven Rh und negativen Pt Anomalien, und eine leicht positive Neigung (Pd/Ir = 1 bis 2). Diese ungewöhnlichen PGE Verteilungsbilder erinnern an die von Sulfiden aus Komatiiten.
Die PGE Verteilung in Serpentiniten ist typisch für leicht verarmte Mantelgesteine, und die assoziierten podiformen Chromitite liegen innerhalb des Bereiches für ophiolitische Chromitite, mit Ausnahme der Anreicherung in Pd und Au.
Lagen von Sulfid-führenden Serpentiniten zeigen relativ hohe Gehalte an Edelmetallen, und PGE-Verteilungsmuster die denen von Co-Arseniderzen sehr ähnlich sind, obwohl sie um eine Größenordnung niedriger liegen. Diese Sulfide dürften Produkte der Remobilisierung primärer magmatischer Sulfide während der Serpentinisierung sein. Die Sulfid-reichen Lagen sind als ein mögliches Ursprungsgestein für die Edelmetalle der Kobalt-Arsenid-Erze von Bou Azzer zu sehen.
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Leblanc, M., Fischer, W. Gold and platinum group elements in cobaltarsenide ores: Hydrothermal concentration from a serpentinite source-rock (Bou Azzer, Morocco). Mineralogy and Petrology 42, 197–209 (1990). https://doi.org/10.1007/BF01162691
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DOI: https://doi.org/10.1007/BF01162691