Survival of whole-rock Sm–Nd isotope system from REE redistribution and mineral-scale isotopic resetting amid hydrothermal alteration in REE-rich Fe-Cu deposit

Abstract

The 147Sm–143Nd isotope system has long been used as a powerful tracer and geochronometer for a wide range of geological process, yet the robustness of Sm–Nd system has been questioned in the presence of multistage hydrothermal alteration. Here we provide robust evidence for the survival of whole-rock Sm–Nd isotopes from REE redistribution and mineral-scale isotopic resetting during multistage fluid infiltrations in the Lala Fe-Cu-REE deposit in Kangdian region, southwestern China.The Lala deposit is hosted in the ∼1.72–1.68 Ga rift-related Hekou Group and contains variable proportions of REE-rich apatite and fluorite in the ores. Both of the apatite and fluorite grains show textures of extensive metasomatic alteration, during which large amounts of REE, especially LREE, were leached out and mostly re-incorporated into newly-formed monazite, bastnäsite, and xenotime. Quantifying REE distribution budget at thin-section scale confirms that the amounts of REE (1583–5403 ppm) leached from primary apatite and fluorite are compatible with those (1636–5526 ppm) incorporated in the newly crystallized REE minerals, indicating that REE are internally redistributed within investigated samples. The whole ores yield an 18-point 147Sm–143Nd errorchon date of ∼1.71 Ga, which is in accordance with the other analogous Fe-Cu deposits in Kangdian region, and supports a ∼1.71 Ga primary mineralization at Lala. By contrast, SHRIMP 232Th–208Pb dating of texturally constrained monazite yield a crystallization age of ∼0.92 Ga, which corresponds well with the combined monazite-bastnäsite-apatite Sm–Nd isochron ages of representative samples, demonstrating a major REE redistribution and Sm–Nd isotopic resetting event at this time. The results suggest that Sm–Nd mobility was limited to within the whole-rock scale or even within the thin-section scale in the vast majority of cases, such that the whole-ore Sm–Nd isotopic system largely remained closed, despite complete resetting of the major REE minerals during younger fluid infiltration events. Our study exemplifies the apparent “decoupling” of Sm–Nd isotopic systems between macro- and microscales, which can advance our understanding of the complex history of hydrothermal systems. Our findings of a major transition (80–96%) in REE repositories also highlight the economic significance of hydrothermal alteration for modifying ore mineral assemblages, which resembles a natural REE refinement process and helps create beneficial REE resources to meet the society increasing demands.