Weathering co-mineralization of placer type ilmenite and ion-adsorption type rare earth elements in Guangxi, China: Nature, origin and exploration implications

The placer type ilmenite and ion-adsorption type rare earth elements (iREEs) deposits are two important weathering-related mineral resources in South China, a global supplier of Ti and REEs critical metals. It was believed that these two deposits are not linked to each other because of their different ore-forming conditions and mechanisms. However, many studies have reported that parts of placer ilmenite deposits, especially for those developed by weathering of intermediate igneous rocks, also contain abundant iREEs resources that have not been noticed before. To elucidate the origin of this interesting co-mineralization phenomenon that is of both scientific and economic significance, we conducted a comparative study of three deposits, including Qinghu (QH), Madi (MD), and Tangcun (TC) from Guangxi Province in the western segment of South China. Investigations showed that although the ore-forming parent rocks from the QH, MD, and TC deposits are intermediate igneous rocks, their corresponding regolith profiles are significantly different in their mineralization characteristics. The diorite-derived QH and tonalite-derived MD deposits showed co-mineralization of ilmenite with iREEs, whereas the pyroxene diorite-derived TC deposit only showed independent mineralization of ilmenite. Evidences from systematic mineralogical and geochemical examinations indicated that the occurrence of ilmenite and iREEs co-mineralization, besides the impact of weathering, may principally regulated by the nature of accessory mineral assemblage in the parent rocks. Specifically, both Ti-bearing (ilmenite, anatase) and REEs-bearing (apatite, titanite) accessory minerals were largely identified in the QH diorite and MD tonalite. Analysis of degree of saprolitization (DOS) and weathering index of granite (WIG) revealed that they have experienced strong weathering, which gave rise to residual accumulation of Ti-bearing minerals (mainly ilmenite) at upper horizons of the regolith profiles, and simultaneously, secondary enrichment of iREEs by eluviation-illuviation process at lower horizons. It explained the ore horizons of placer ilmenite and iREEs developed symbiotically but show somewhat dislocation within the regolith. In contrast, the TC pyroxene diorite was found to be rich only in Ti-bearing minerals but poor in REEs-bearing minerals. It is conductive to develop weathering mineralization of ilmenite rather than iREEs in terms of ore source. Also, there is rich iron oxides and hydroxides in the pyroxene diorite-derived regolith, which might has inhibited REEs eluviation-illuviation and act as the other reason for absence of iREEs mineralization in the TC profile. This study not only provides a genetic model to understand the co-mineralization of placer ilmenite and iREEs ores developed by weathering of intermediate igneous rocks, but also highlights the importance of a comprehensive evaluation in future prior to exploration and resource utilization.