Iron oxide copper–gold (IOCG) deposits worldwide have been documented to be variably enriched in REEs in addition to Fe, Cu and Au metals, but the key factors controlling REE enrichments have long been poorly constrained. In this study, we conducted mineralogical and geochemical investigations on the timing and genesis of the REE enrichments in the Lala and Dahongshan deposits, the two largest IOCG deposits in the Kangdian metallogenic province, SW China. Both deposits have comparable nature of hosting rocks and a paragenetic sequence composed of early magnetite, accompanied with regional sodic alteration, and late Cu-sulfide mineralization, associated with K-Fe alteration, but exhibit different styles and grades of REE enrichments. The REE enrichments in Lala is close to industry-level, presenting as monazite with subordinate bastnaesite, allanite, and apatite of which apatite is mainly associated with the Fe mineralization stage, while others are co-existing with the Cu mineralization stage. In contrast, REE enrichments in Dahongshan is dominated by apatite that is mainly associated with magnetite of the Fe mineralization stage. Our new U-Pb dating of monazite from the Lala deposit yields a Neoproterozoic (~850 Ma) age. Such an age is slightly younger than previously obtained molybdenite Re-Os and allanite U-Pb ages of ~ 1.05 Ga, indicating that it should represent a fully or partially reset age that is related to regional Neoproterozoic (890–740 Ma) overprint event. On the other hand, previous zircon U-Pb dating show that the REE enrichments in the Dahongshan deposit was formed at ~ 1.66 Ga.

Our in-situ Nd isotopic analyses show that the Lala monazite has εNd(t) values (calculated at 1080 Ma) ranging from -7.5 to -2.9, comparable to those of hosting rocks (7.2–0.3 at 1080 Ma) and partially overlapping contemporaneous felsic intrusions (-7.2 to 4.4), indicating hybrid sources of REEs from hosting rocks and/or coeval magmas. On the other hand, apatite grains in the Dahongshan deposit have εNd(t) values (calculated at 1660 Ma) ranging from -17.8 to -3.5, broadly similar to Archean besement rocks (-16.7 to -7.0 at 1660 Ma) in the Yangtze Block, but different from hosting rocks (2.9 to 4.6 at 1660 Ma) and contemporaneous mafic rocks (-0.2 to 4.0). In combination with textural observations (e.g., monazite-bearing apatite in Lala), this study further confirmed that different nature of Cu mineralizing fluids is the key factor controlling mobilization and leaching of REEs from the hosting rocks with similar lithologies. Available studies regarding fluid inclusion indicated that the fluids in both deposits are broadly similar, rich in Cl^- and CO₂ (e.g., HCO₃^- and CO₃^2-). However, the presence of abundant fluorite in the Lala deposit (absent in the Dahongshan deposit) implies that the fluids in Lala should be distinctly rich in F. This conclusion is also supported by fact that the apatite in Lala has F and F/Cl values much higher than those of Dahongshan. Therefore, the relatively F-rich nature, which was generally considered to be more capable for breakdown of REE minerals, plays a key role for the extensive REE mobilization and mineralization in the Lala deposit. This study highlights that variable REE enrichments in IOCG deposits were likely mainly controlled by REE budgets of wall rocks and the nature of mineralizing fluids. (C) 2021 Elsevier B.V. All rights reserved.

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