Two episodes of carbonatite-related rare-earth element mineralization in the Nam Xe deposit, northwestern Vietnam: Implications for ore genesis and regional metallogeny

Abstract

Carbonatite-related rare-earth element (REE) deposits are developed in large areas of southwestern China and northwestern Vietnam on the SW margin of the South China block. The ages of REE deposits and their host carbonatite-alkaline rocks in southwestern China have been tightly constrained in the 35–10 Ma interval, but those in northwestern Vietnam remain poorly understood. In this paper, we present new Th–Pb dating results to constrain the timing of mineralization of the Nam Xe REE deposit in northwestern Vietnam, and to reveal its possible relationship to the well-studied REE deposits in southwestern China. The Nam Xe deposit consists of three types of REE ores: Type 1, fine-grained disseminated ores; Type 2, coarse-grained, taxitic ores; and Type 3, REE veinlets. The mineral assemblages and their close relations to hydrothermal alteration, combined with the trace element patterns of the ore minerals, indicate a hydrothermal origin for all the three ore types. Laser ablation-inductively coupled plasma-mass spectrometry Th–Pb dating of monazite yielded weighted-mean Th–Pb ages of 45–44 Ma for type 1 ores and 35–32 Ma for type 2 and 3 varieties, whereas the LA-ICPMS dating of bastnäsite has ages of 35–31 Ma for types 2 and 3, revealing two REE mineralization events at 45–44 and 35–31 Ma in the Nam Xe deposit. The age of type 1 ores overlaps the main-collision period of India-Asia collisional orogeny (65–41 Ma) which confirms carbonatite-related REE mineralization could form in the main-collisional geological setting. The ages of types 2 and 3 ores are consistent with those of the Mianning–Dechang REE metallogenic belt in southwestern China that formed under a late-collision extensional setting (40–26 Ma). Combined with existing geochronological data, results presented here suggest that the Nam Xe REE deposit formed both in the main- and late-collisional settings and represents the southern extension of the Mianning–Dechang REE metallogenic belt.