Abstract: The Ximeng-Lancang area in southwestern Yunnan resides in the southern section of the Nujiang-Lancangjiang-Jinshajiang orogenic belt in Southwest China. The 1∶50, 000 geochemical soil survey revealed 24 rare-earth-element (REE) geochemical anomalies, which are primarily distributed in the Carboniferous Pingzhang Formation mafic volcanic rocks and the Carboniferous-Permian Yutangzhai Formation sedimentary carbonate rocks within and near the Changning-Menglian deep fault zone. Furthermore, the AP00 REE geochemical anomalies ranking high in the evaluation were analyzed in detail through a 1∶10, 000 geochemical soil survey, a 1∶10, 000 special geological survey, and light-duty prospecting engineering in mountainous areas. A new type of REE ores in weathering crusts has been first discovered in sedimentary carbonate strata, with preliminarily estimated REE resources reaching a medium scale, suggesting a prospecting breakthrough. This finding shows a new prospecting approach, which can be referenced for similar research. As revealed by a comprehensive analysis of the regional geological and geochemical settings and the data of AP00 REE anomalies, the AP00 REE ores in weathering crusts have undergone a gradual enrichment and mineralization process involving four different geological processes, suggesting polygenetic compound REE ores. Considering the low leaching efficiency of AP00 REE ores and significant structural (magmatic) superimposed mineralization of the heavy REE yttrium, it is inferred that yttrium-dominated primary REE ores might exist in the deep part, implying high potential for heavy-REE ores.