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Bastnäsite contains considerable amounts of U and Th and has been widely used for U-Th-Pb dating. Hydrothermal alteration of bastnäsite is common in nature but its effects on U-Th-Pb dating are not currently well constrained. Hence the significance of U-Th-Pb ages obtained from altered bastnäsite cannot be evaluated. Here, we present a detailed geochronologic as well as micro- and nano-scale mineralogical study of altered bastnäsite in a Mo-REE deposit, Central China. The original bastnäsite grains were confirmed to have crystalized at 208 Ma but were variably overprinted by a hydrothermal event at 150 Ma. They commonly exhibit typical replacement textures that appear to have formed from a coupled dissolution-reprecipitation process, i.e., a primary unaltered domain surrounded by a porous altered domain. Micro- and nano-scale mineralogical observations strongly suggest that during the coupled dissolution-reprecipitation process, non-radiogenic (common) Pb was incorporated into the altered domains in the form of nanoscale galena inclusions. Such incorporation (even minor) has significantly affected the 206Pb/238U and 207Pb/206Pb ratios due to the low contents of U and its daughter isotopes in bastnäsite, resulting in highly variable, discordant U-Pb dates for the altered domains. In contrast, incorporation of the non-radiogenic Pb has very limited effects (<5%) on the Th-Pb system due to the remarkably high contents of Th and radiogenic 208Pb in bastnäsite. Instead, the scattered 208Pb/232Th ages (208 to 150 Ma) of the altered domains were essentially affected by incomplete replacement, and thus can be used to approximate the lower age limit of the primary hydrothermal activity or the upper age limit of the secondary hydrothermal activity. The results from this study highlight that because of the different orders of magnitude between the U and Th contents in bastnäsite, the mobilization of radiogenic and non-radiogenic Pb during alteration may have significantly different impacts on the U-Pb and Th-Pb systems. Therefore, the two systems should be treated separately during the dating of bastnäsite resulting from secondary hydrothermal events. [ABSTRACT FROM AUTHOR]

The ~1.74 Ga Damiao anorthosite complex, North China, is composed of anorthosite and leuconorite with subordinate melanorite, mangerite, oxide-apatite gabbronorite, perthite noritic (i.e., jotunitic) and ferrodioritic dykes. The complex hosts abundant vein-, pod- and lens-like Fe-Ti-P ores containing variable amounts of apatite (10-60 modal%) and Fe-Ti oxides. In addition to Fe-Ti-P ores, there are also abundant Fe-Ti ores which are closely associated with Fe-Ti-P ores in the deposit. Most of Fe-Ti-P ores are dominated by Fe-Ti oxides and apatite, devoid of silicate minerals, mineralogically similar to the common nelsonites elsewhere. In contrast, Fe-Ti ores are dominated by Fe-Ti oxides with minor apatite ( Keywords Differentiation * Immiscibility * Massif-type anorthosite Nelsonite North China, Introduction Nelsonites are commonly associated with Proterozoic massif-type anorthosites, and are important sources of Fe, Ti and P (Watson and Taber 1913; Force 1991; Ashwal 1993; Darling and Florence 1995). [...]