Nadeau, Olivier; Stevenson, Ross et Jébrak, Michel
(2014).
« The Archean magmatic-hydrothermal system of Lac Shortt (Au-REE), Abitibi, Canada: Insights from carbonate fingerprinting ».
Chemical Geology, 387, pp. 144-156.
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Résumé
Trace element and isotopic data from the Lac Shortt (Abitibi, Canada) gold and rare earth element (REE) deposit sheds light on the cogenetic association between alkali magmatism (syenites and lamprophyres), carbonatation (magmatic and carbothermal) and gold – REE mineralization. Magmatic and carbothermal carbonates from comagmatic lithologies from the Lac Shortt gold-REE deposit were analyzed for major, minor and trace elements concentrations as well as for their 8613Sr/18 Sr0, δCandδ O isotopic ratios. Carbonates from Lac Shortt have δ13C values ranging from −4.89 to 2.38 ‰ VPDB and δ18 O values ranging from 8.28 to 15.02. The lightest values are found in carbonate from the carbonatites and the heaviest values are associated with the ore-carbonates.
Carbonates from the various lithologies lie between these two extremes.
Carbonate fingerprinting 8786Sr/Sr ratios from the carbonates cluster tightly around 0.7013–0.7016, i.e., depleted Archean mantle signatures, although some have slightly more radiogenic values, from 0.7026 to 0.7029. The small variation in the strontium isotope ratios compared to
the C and O isotopic values suggests that carbonate anions (CO32−) were decoupled from the cations during hydrothermal alteration. Cations such as Sr2+2+and Ca in carbonates from the high grade ore and from magmatic and hydro-carbothermal carbonatite appear to originate from the Late-Archean, depleted mantle. Since gold is a cation and is strongly associated with carbonatite magmatism and associated carbo-hydrothermal carbonatation, we suggest gold too has a depleted mantle origin. The carbonates are enriched in mobile high field strength elements (HFS; Ba, Sr) elements, depleted in immobile HFS elements (Th, U, Ta, Nb, Zr, Ti) and have enriched- to slightly depleted REE patterns with flat to strong light-REE enrichments. These trace element concentrations support the fact that all the lithologies were comagmatic and that the ore itself was genetically
related to these lithologies. Based on present day knowledge of Archean tectonics of Abitibi, post collision extension tectonics and mantle delamination, isotope geochronology, on the magmatic-hydrothermal system
of Lac Shortt itself and on the present paper's carbonate fingerprinting, we propose that Lac Shortt calcalkaline dioritic magmatism-hydrothermalism was followed by alkaline syenitic, and eventually carbonatitic magmatism-hydro-carbothermalism, and that gold and REE originated from the carbonatite magmaticcarbo-hydrothermal system and its mantle roots, which agrees with mass balance calculations. The depletedmantle-rooted, carbonatite exsolved a hydro-carbothermal fluid enriched in REE and gold which fenitized and
oxidized the syenite and out of which the gold and REE mineralization was deposited.
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