Abstract Bull Hill is a carbonatite diatreme within the Paleogene Bear Lodge Carbonatite Complex in Wyoming, USA. Rare earth element (REE)-bearing carbonate, fluorocarbonate, phosphate, and oxide minerals occur within near-vertical carbonatite dikes on the western margin of Bull Hill. Changes in mineralogy and REE concentrations with depth are ascribed mainly to late-stage magmatic-hydrothermal and supergene alteration. Approximately 35 m of drill core from Bull Hill was analyzed and encompasses least altered, weakly weathered, and moderately weathered carbonatite. The least altered carbonatite contains magmatic burbankite, typically as inclusions within Mn-rich calcite (stage I). Secondary REE-bearing minerals, which pseudomorphically replaced unidentified hexagonal phenocrysts, include ancylite, bastnäsite with synchysite/parisite, and an unidentified Sr-Ca-REE-phosphate (stage II). These replacive minerals generated small amounts of incipient porosity (~7–8%) and are largely stable in the lower portion of the weathering profile. Progressive weathering (stages III and IV) of the carbonatite involved the oxidation of pyrite to iron oxides and iron hydroxides, dissolution of calcite and strontianite, and the replacement of Mn-rich calcite by manganese oxides. These mineralogical changes resulted in an ~40% porosity gain in the core studied here. The volumetric concentration of weathering resistant REE-bearing minerals resulted in REE enrichment from an average of 5.4 wt % in the least weathered carbonatite to an average of 12.6 wt % in moderately weathered carbonatite, and to an overall increase in REE ore tenor of two to three times compared to the least altered carbonatite. Isocon plots confirm the increased concentration of REEs in the weathered carbonatite and demonstrate that REEs, along with TiO2, Ta, Nb, Zr, and Hf, were conserved in the lower weathered zone.

中文翻译：

---

美国怀俄明州贝尔小屋布尔山碳酸岩风化剖面中的稀土元素富集

摘要 Bull Hill 是美国怀俄明州古近纪 Bear Lodge 碳酸盐岩复合体中的碳酸盐岩。含稀土元素 (REE) 的碳酸盐、氟碳酸盐、磷酸盐和氧化物矿物出现在 Bull Hill 西缘的近垂直碳酸盐岩脉内。矿物学和稀土元素浓度随深度的变化主要归因于晚期岩浆热液和表生蚀变。对来自 Bull Hill 的大约 35 m 钻芯进行了分析，其中包括变化最少、风化较弱和中风化的碳酸盐岩。蚀变最少的碳酸盐岩包含岩浆水沸石，通常作为富锰方解石中的夹杂物（阶段 I）。次生含 REE 矿物，以假晶形式取代了未识别的六方斑晶，包括长长石、氟碳铈矿和合辉石/硅镁石、和一种未鉴定的 Sr-Ca-REE-磷酸盐（第二阶段）。这些替代矿物产生少量初始孔隙度（~7-8%），并且在风化剖面的下部基本稳定。碳酸岩的渐进风化（阶段 III 和 IV）涉及黄铁矿氧化成氧化铁和氢氧化铁，方解石和锶石溶解，以及用氧化锰替代富含锰的方解石。这些矿物学变化导致此处研究的岩心孔隙度增加了约 40%。耐风化的含 REE 矿物的体积浓度导致 REE 富集从最小风化碳酸盐岩中的平均 5.4 wt% 到中等风化碳酸盐岩中的平均 12.6 wt%，并使 REE 矿石品位总体增加 2 到是变化最少的碳酸岩的三倍。