Tantalum and niobium mineralization from F- and Cl-rich fluid in the lepidolite-rich pegmatite from the Renli deposit in northern Hunan, China: Constraints of fluid inclusions and lepidolite crystallization experiments

Abstract

Lepidolite-type pegmatite usually produces tantalum–niobium (Ta–Nb) resources; however, the role of the hydrothermal fluid in the Ta–Nb mineralization remains controversial. The pegmatite dike No. 5 is a lepidolite-rich dike in the Ta–Nb pegmatite deposit in Renli, Hunan, China, with a high-grade Ta2O5 in the lepidolite–quartz core. The fluid inclusions with primary features in the quartz from the core have a relatively high Ths (~270–290 °C) and salinity (~10 wt% NaCleq). The mineral features such as occurrences of pezzottaite and fluornatromicrolite, as well as the assemblage of lepidolite, quartz, and albite, further confirm that the fluid inclusions were entrapped in a Na-, Li-, Cs-, F-, and Cl-rich fluid from which quartz and lepidolite crystallized. These fluid inclusions were inferred to be trapped between 640 °C and 530 MPa and 550 °C and 350 MPa, based on the fluid inclusion isochores and the lepdilite crystallization temperature–pressure (T–P) conditions that were determined via lepidolite crystallization experiments in aqueous solutions with a high fHF and a salinity of 10 wt% NaCl in a hydrothermal diamond-anvil cell. Although the P–T conditions are in accordance with the stability field of spodumene, no spodumene was observed in the No. 5 dike because of the high fHF in the fluid. Such Li-, Cs-, F-, Cl-rich fluids could be an indicator of the final layer liquid in the model of constitutional zone refining or B-type fluid/melt in the immiscibility model. Moreover, it could dissolve the previous magmatic Nb–Ta minerals in the form of (Li, Na, Cs)[Ta(Nb)O(F, Cl)4] with Ta–Nb remaining in the aqueous solution until the deposition of Ta-Nb minerals during the crystallization of lepidolite and albite in lepidolite-qaurtz core in No.5 dike in the Renli deposit.