Ultrapotassic plutons as a source of uranium of vein-type U-deposits (Moldanubian Zone, Bohemian Massif): insights from SIMS uraninite U–Pb dating and trace element geochemistry

IF 4.4 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Mineralium Deposita Pub Date : 2024-04-11 DOI:10.1007/s00126-024-01263-6
Martin Kubeš, Jaromír Leichmann, Vojtěch Wertich, Renata Čopjaková, Markéta Holá, Radek Škoda, Bohdan Kříbek, Julien Mercadier, Michel Cuney, Etienne Deloule, Andreï Lecomte, Ewa Krzemińska
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Abstract

The Bohemian Massif hosts significant hydrothermal U-deposits associated with shear zones in the high-grade metamorphic basement. But there is a lack of evidence of a genetic link between mineralization and U-fertile igneous rocks. This contribution provides constraints on the major U source of the vein-type U-deposits, the timing of ore formation and the metallogenetic model. The anomalous trace element signatures of the low-temperature hydrothermal deposits (high Zr, Y, Nb, Ti, ∑REE) and their close spatial relation with ultrapotassic rocks of the durbachite series point to a HFSE and REE enriched source rock. The durbachites have high U content (13.4–21.5 ppm) mainly stored in magmatic uraninite and other refractory minerals (e.g., thorite, zircon, allanite) that became metamict over a time interval sufficient to release U from their crystal structure, as suggested by the time gap between emplacement of the durbachites (EMP uraninite U–Pb age ~ 338 Ma) and hydrothermal activity (SIMS uranium ore U–Pb age ~ 270 Ma). Airborne radiometric data show highly variable Th/U ratios (1.5–6.0), likely reflecting a combination between (1) crystallization of magmatic uraninite, (2) hydrothermal alteration, and (3) leaching and mobilization of U along NW–SE-trending fault zones, manifested by elevated Th/U values in the radiometric map. The presence of rare magmatic uraninite in durbachites suggests almost complete uraninite dissolution; EMP imaging coupled with LA-ICP-MS analyses of refractory accessory phases revealed extensive mobilization of U together with HFSE and REE, providing direct evidence for metal leaching via fluid-driven alteration of radiation-damaged U-rich minerals. The large-scale HFSE and REE mobilization, demonstrated by the unusual trace element signatures of the U-deposits, was likely caused by low-temperature (270–300 °C), highly alkaline aqueous solutions containing F-, P-, and K-dominated complexing ligands. The first SIMS U–Pb age of 270.8 ± 7.5 Ma obtained so far for U-mineralization from the Bohemian Massif revealed a main Permian U mineralizing event, related to crustal extension, exhumation of the crystalline basement, and basin formation, as recorded by U–Pb apatite dates (280–290 Ma) and AFT thermal history models of the durbachites. The Permo-Carboniferous sedimentary cover probably represented a source of oxidized basinal brines infiltrating the basement-hosted durbachite plutons and triggering massive metal leaching. The interaction between basin-derived brines and durbachites resulted in significant modification of the chemical composition of the hydrothermal system (K and F release during biotite chloritization, P liberation through monazite alteration), leading to the formation of ore-bearing fluids responsible for the metallogenesis of the basement-hosted unconformity-related U-deposits in shear zones in the Bohemian Massif.

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作为脉型铀矿床铀源的超基性块岩(波希米亚山丘摩尔多瓦区):SIMS铀矿石U-Pb定年和痕量元素地球化学的启示
波希米亚丘(Bohemian Massif)拥有大量与高品位变质基底剪切带有关的热液铀矿床。但缺乏证据表明矿化与富含铀的火成岩之间存在遗传联系。这篇论文对矿脉型铀矿床的主要铀源、矿石形成的时间和成矿模式提供了约束。低温热液矿床的异常微量元素特征(高Zr、Y、Nb、Ti、∑REE)及其与杜尔拉奇岩系列超生岩的密切空间关系表明,矿床的源岩富含高频闪长岩和REE。杜尔巴赫岩的铀含量很高(13.4-21.5 ppm),主要储存在岩浆铀矿石和其他难熔矿物(如透辉石、锆石、阳起石)中,这些矿物在一定的时间间隔内发生变质,足以从其晶体结构中释放出铀,杜尔巴赫岩的成因(EMP铀矿石U-Pb年龄~338 Ma)与热液活动(SIMS铀矿石U-Pb年龄~270 Ma)之间的时间差也表明了这一点。机载辐射测量数据显示 Th/U 比值变化很大(1.5-6.0),很可能反映了(1)岩浆铀矿的结晶,(2)热液蚀变,以及(3)沿西北-东南走向断层带铀的沥滤和移动,在辐射测量图中表现为 Th/U 值升高。杜拉奇岩中存在稀有的岩浆铀矿石,这表明铀矿石几乎完全溶解;EMP 成像与难熔附属相的 LA-ICP-MS 分析相结合,揭示了铀与高频闪锌矿石和稀土元素的广泛迁移,为通过流体驱动的辐射损伤富含铀矿石的蚀变进行金属沥滤提供了直接证据。铀沉积物不寻常的痕量元素特征表明,大规模的高频闪锌矿和稀土元素移动很可能是由低温(270-300 °C)、高碱性水溶液造成的,其中含有以 F、P 和 K 为主导的络合配体。迄今为止从波希米亚丘获得的第一个 SIMS U-Pb 年龄(270.8 ± 7.5 Ma)揭示了一个主要的二叠纪铀矿化事件,该事件与地壳延伸、结晶基底的掘起和盆地形成有关,U-Pb 磷灰石日期(280-290 Ma)和杜巴赫岩的 AFT 热历史模型记录了这一事件。二叠纪石炭纪沉积覆盖层可能是氧化基底卤水的来源,这些卤水渗入基底托举的杜拉奇岩柱体,引发了大规模的金属沥滤。盆地卤水和杜尔拉奇岩之间的相互作用导致热液系统的化学成分发生重大变化(在生物绿泥石化过程中释放出 K 和 F,通过独居石蚀变释放出 P),从而形成含矿流体,导致波希米亚丘剪切带中基底托管的、与地貌不符的铀矿床的金属生成。
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来源期刊
Mineralium Deposita
Mineralium Deposita 地学-地球化学与地球物理
CiteScore
11.00
自引率
6.20%
发文量
61
审稿时长
6 months
期刊介绍: The journal Mineralium Deposita introduces new observations, principles, and interpretations from the field of economic geology, including nonmetallic mineral deposits, experimental and applied geochemistry, with emphasis on mineral deposits. It offers short and comprehensive articles, review papers, brief original papers, scientific discussions and news, as well as reports on meetings of importance to mineral research. The emphasis is on high-quality content and form for all articles and on international coverage of subject matter.
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