Tourmaline and monazite chemistry reflects magmatic-hydrothermal evolution of the Xianghualing W-Sn-Nb-Ta (Li, Be) pegmatite and metasomatite, South China

Abstract

The Nanling Metallogenic Belt, one of the world's largest rare-metal mineralization provinces, contains the Laiziling W-Sn-Nb-Ta rare-metal deposit. Despite its economic importance, the Li and Be mineralization and associated hydrothermal history of this deposit have not been studied in detail and remain controversial. This study presents the first in situ UPb dating and Nd isotopic analysis of monazites, as well as chemical composition and B isotopic data of tourmalines, from Laiziling pegmatites and metasomatites. LA-ICP-MS UPb geochronology yields crystallization ages of 152.0 ± 1.1 Ma for magmatic monazite and 151.9 ± 3.0 Ma and 151.2 ± 2.1 Ma for hydrothermal monazites. These results indicate that the hydrothermal stage was synchronous with pegmatite emplacement, and that pegmatite crystallization was related to hydrothermal activity. Variable Eu anomalies in monazite suggest an increase in environmental oxygen fugacity during the hydrothermal stage. The εNd(t) values of magmatic monazite (−7.49 to −6.82) are slightly lower than those of altered monazite (−7.24 to −6.51), indicating a slight disturbance of the Nd isotopic system by hydrothermal processes. The tourmalines exhibit oscillatory zoning from core to rim (Tur-I to Tur-VI) with δ¹¹B values ranging from −13.6 to −11.7 ‰. This observation suggests that tourmaline was derived from a single continental crustal source. The δ¹¹B of tourmaline shows a gradual decrease from cores to rims. The variation in B isotopes and chemical composition of tourmaline may be related to the mixed of the original igneous components in the earlier stage of metasomatism. LA-ICP-MS mapping of the cross and longitudinal sections of tourmaline shows elemental variations during its crystallization, with Tur-IV used as an indicator of magma evolution into the hydrothermal stage. After entering the hydrothermal stage, exchange vectors gradually changed from □Al(NaMg)₋₁ to MgFe₋₁, with the tourmaline type transitioned from schorl to Mg-foitite. Although rare metal mineralization and pegmatite emplacement were synchronous, the multiple stages of magma evolution exhibit differences in mineralization. The mineralization of NbTa during the magmatic period has been confirmed previously. Principal component analysis (PCA) was used to predict the geochemical behavior of Li-Be-W-Sn and to determine the chemical controls on tourmaline crystallization at different stages of magmatic evolution. Enrichment and precipitation of Li usually occur during the magmatic stage, while Be-W-Sn mineralization is typically related to late hydrothermal alteration. However, the Laiziling deposit is characterized by Li mineralization synchronously with pegmatite emplacement, and by Be-W-Sn mineralization with hydrothermal alteration after this emplacement event. Our findings provide new insights into the formation processes of rare-metal mineral deposits.