风化岩中高品位 REE 的积累:南澳大利亚卡彭达铜矿富磷灰石矿脉超生蚀变的启示

IF 4.4 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Mineralium Deposita Pub Date : 2024-05-21 DOI:10.1007/s00126-024-01283-2
Tobias G. Bamforth, Fang Xia, Caroline J. Tiddy, Ignacio González-Álvarez, Joël Brugger, Si-Yu Hu, Louise E. Schoneveld, Mark A. Pearce, Andrew Putnis
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引用次数: 0

摘要

位于南澳大利亚的地层和脉床 Kapunda 铜矿床含有一条溶蚀热液矿脉,其中稀土氧化物(TREO)总量为 12.37 wt.%。通过 X 射线衍射、扫描电子显微镜、同步辐射 X 射线荧光显微镜和电子反向散射衍射对该矿脉进行了分析,以了解在强烈风化期间对高品位稀土氧化物积累的控制。岩石学评估表明,含磷灰石-方解石-铝硅酸盐的原岩已转变为铁氧化物、高岭石和 REE 磷酸盐矿物的超生集合体,其中包括菱锰矿(Ce)、独居石(Ce)和萤石(Ce)。风化流体的逐渐酸化促进了这种转变,具体表现为1)自生铁氧体和高岭石的结晶度增加,导致 REE 解吸;2)自生 REE 磷酸盐的纹理演变和晶粒大小增加,从纳米级结晶到针状,再到微尺度的六方棱柱;3)REE 磷酸盐的后期溶解;以及 4)透辉石被叶腊石取代,叶腊石中的硫酸盐成分来源于近硫化物矿物的氧化和风化。方解石半月板的保存表明,除了缓冲 pH 值的碳酸盐矿物消耗殆尽之外,硫化物的溶解还促进了酸的生成。研究结果表明,高酸性风化流体可能会促进REE的移动或REE在风化岩中的积累。当主岩含有大量沉积配体(即磷灰石形式的磷酸盐)时,在酸性超成岩条件下的高品位 REE 积累会被优先考虑,这些配体在强烈风化过程中很容易被沥滤。因此,勘探公司应该对任何严重风化的磷酸盐岩中的 REE 进行例行化验,因为磷酸盐矿物是 REE 累积的地球化学陷阱。
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High-Grade REE accumulation in regolith: Insights from supergene alteration of an apatite-rich vein at the Kapunda Cu mine, South Australia

The stratiform and vein-hosted Kapunda Cu deposit in South Australia contains a saprolitized hydrothermal vein with 12.37 wt.% total rare earth oxide (TREO). The vein was analyzed by X-ray diffraction, scanning electron microscopy, synchrotron-based X-ray fluorescence microscopy and electron backscatter diffraction to understand the controls that govern high-grade REE accumulation during periods of intense weathering. Petrological assessments indicate the transformation of an apatite-calcite-aluminosilicate-bearing protolith to a supergene assemblage of Fe-oxides, kaolinite and REE-phosphate minerals that include rhabdophane-(Ce), monazite-(Ce) and florencite-(Ce). This transformation was facilitated by progressive acidification of the weathering fluid, which is indicated by: 1) the increasing crystallinity of authigenic Fe-oxides and kaolinite, which led to REE desorption; 2) the textural evolution and increase in grain size of authigenic REE-phosphates from nanoscopic crystallites, to acicular needles, to micro-scale hexagonal prisms; 3) the late dissolution of REE-phosphates; and 4) the replacement of goethite by jarosite, whose sulfate component originated from the oxidation and weathering of proximal sulfide minerals. Alongside the depletion of pH-buffering carbonate minerals that are indicated by the preservation of calcite menisci, this sulfide dissolution also facilitated acid generation. Results illustrate how highly acidic weathering fluids might facilitate either REE mobilization or REE accumulation in regolith. High-grade REE accumulation under acidic supergene conditions is prioritized when the host-rock contains a significant source of depositional ligands (i.e., phosphate in the form of apatite) that can be readily leached during intense weathering. Exploration companies should therefore assay routinely for REEs in any heavily weathered phosphatic rock, due to the observed efficiency of phosphate minerals as geochemical traps for REE accumulation.

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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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