Monazite geochronology and geochemistry constraints on the formation of the giant Zhengchong Li-Rb-Cs deposit in South China

Abstract

The giant Zhengchong Li-Rb-Cs deposit (ore resource: 270 k tonnes (t) Li2O @ 0.557 wt%, 110 kt Rb2O @ 0.225 wt%, and 6 kt Cs2O @ 0.013 wt%) in South China is associated with quartz-zinnwaldite-topaz rock, which is composed mainly of quartz, zinnwaldite and topaz with a porphyritic texture. However, the genetic link between these rocks and their Jiuyishan granite batholith host is unclear, and the magmatic-hydrothermal process involved in the rare-metal mineralization remains enigmatic. In this study, we analyzed the in-situ U-Pb age and mineral chemistry of monazite, a ubiquitous accessory phase at Zhengchong, to reconstruct the precise geochronological framework of this highly-fractionated felsic magmatic system, and the origin of the Li-Rb-Cs-rich quartz-zinnwaldite-topaz rock. Based on petrographic observations and back-scattered electron (BSE) imaging, monazite crystals in different rocks at Zhengchong have the following characteristics: (1) Mnz-I is mainly BSE-zoned monazite disseminated in granite porphyry, coexisting with zircon or included in zinnwaldite, (2) Mnz-II consists of relatively fine-grained BSE-zoned or homogeneous monazite distributed in feldspar phenocryst-bearing quartz-zinnwaldite-topaz rock, (3) Mnz-III is BSE-zoned monazite with embayed texture in the core, distributed in coarse-grained quartz-zinnwaldite-topaz rock, and (4) Mnz-IV is homogenous, BSE-gray monazite from fine-grained porphyritic quartz-zinnwaldite-topaz rock (as the main orebodies). All monazite crystals are characterized by high Th content, indicating magmatic origin. The LA-ICP-MS monazite U-Pb ages indicate granitic magmatism and mineralization cluster around 150 Ma, implying a close genetic link between the granite porphyry host and the quartz-zinnwaldite-topaz rock. We compared the chondrite-normalized REE distribution patterns and principal component analysis and partial least squares-discriminant analysis of the monazite analyzed, and suggested that the BSE-zoned Mnz-I, II and III are likely of the same type, significantly different from the homogenous Mnz-IV. The gray rims (under BSE) of Mnz-I, II and III are similar to Mnz-IV, and differ markedly from the bright core (under BSE) in their lower HREE and ThO2, but higher Eu and Eu/Eu* contents. When combined with the whole-rock composition data, the monazite compositional variation cannot be attributed to rapid changes in magma composition, depressurization, and increased water content, but rather to the increasing oxygen fugacity and the decreasing temperature and Si activity, accompanied by the elevated fractionation of granitic magma. Thus, the variation in monazite composition reflects the formation of Zhengchong Li-Rb-Cs deposit, which may be the result of high magmatic differentiation of fluorine-rich A-type magmas. Although multistage rare metal metallogenic events have been identified in the Nanling region, it is suggested that the Late Jurassic Zhengchong Li-Rb-Cs mineralization occurred within a short duration (<1 million years). Overall, the monazite chemical and geochronological signatures are good pathfinders for magmatic evolution and Li-Rb-Cs mineralization in the Jiuyishan complex, which could be used as a potential prospecting tool for highly-evolved granite-related rare metal deposits.