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.

南澳大利亚的层状和脉状 Kapunda 铜矿床含有腐泥土热液脉，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、基于同步加速器的 X 射线荧光显微镜和电子背散射衍射对矿脉进行了分析，以了解强烈风化期间控制高品位 REE 积累的控制因素。岩石学评估表明，含磷灰石-方解石-铝硅酸盐的原岩转变为铁氧化物、高岭石和稀土磷酸盐矿物的表生组合，其中包括弹石烷-(Ce)、独居石-(Ce)和荧光石-(Ce)。风化液的逐渐酸化促进了这种转变，这表明：1）自生铁氧化物和高岭石的结晶度增加，导致稀土元素解吸； 2）自生稀土磷酸盐的结构演变和晶粒尺寸的增加，从纳米微晶到针状针，再到微米级六角棱柱； 3）稀土磷酸盐的后期溶解； 4）黄钾铁矾取代针铁矿，黄钾铁矾的硫酸盐成分来源于近端硫化物矿物的氧化和风化。除了方解石弯液面保存所表明的 pH 缓冲碳酸盐矿物的消耗之外，这种硫化物溶解也促进了酸的产生。结果说明了高酸性风化流体如何促进风化层中稀土元素的流动或稀土元素的积累。当主岩含有重要的沉积配体来源（即磷灰石形式的磷酸盐）时，酸性表生条件下的高品位稀土元素积累优先，这些配体在强烈风化过程中很容易被浸出。 因此，由于观察到磷酸盐矿物作为稀土元素积累的地球化学陷阱的效率，勘探公司应定期检测任何重风化磷岩中的稀土元素。