Origin of Ligongling ore-bearing breccia pipe in the Yangchuling porphyry W-Mo deposit, South China: Constraints from apatite chemistry and U–Pb dating

Abstract

The Yangchuling district, located within the Jiangnan tungsten belt (JNB), hosts a porphyry-style W–Mo deposit that includes an ore-bearing breccia pipe at Ligongling. This breccia pipe is situated at the contact between granodiorite and the Neoproterozoic Shuangqiaoshan Group metasedimentary rocks. The breccia exhibits a complex assemblage of clasts, including granite porphyry, greisen, siltstone, and weakly altered granodiorite. The evolution of the Ligongling breccia pipe can be subdivided into three stages: a pre-ore stage (Stage I), a breccia-type ore stage (Stage II), and a vein-type ore stage (Stage III). During Stages II and III, scheelite and molybdenite occur as fine-grained disseminations within both the mineralized breccias and associated hydrothermal veins.

Apatite is ubiquitous in various lithologies of the deposit and serves as a sensitive recorder of magmatic and hydrothermal processes. In this study, we present a comprehensive investigation of apatite from the Ligongling breccia pipe, including petrographic observations, In situ U–Pb geochronology, and major and trace element geochemistry. Based on textural and mineralogical characteristics observed under optical microscopy, scanning electron microscopy (SEM), and cathodoluminescence (CL), apatite grains were classified into six types: Ap1 and Ap2 from Stage I; Ap3 and Ap4 from Stage II; and Ap5 and Ap6 from Stage III.

Major and trace element compositions of all apatite types were analyzed using electron probe microanalysis (EPMA) and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS). Total rare earth element (REE) concentrations in apatite systematically decrease from Stage I to Stage III. REE distribution patterns indicate that hydrothermal fluids responsible for ore formation inherited magmatic REE signatures from the granitic source. Apatite associated with W mineralization within the Ligongling granitoids is characterized by low Cl and high F contents, consistent with a F-rich magmatic–hydrothermal system. In situ U–Pb dating of hydrothermal Ap6 apatite from Stage III vein-type mineralization yielded an age of 147.5 ± 4.8 Ma (MSWD = 0.26; n = 17), interpreted as the timing of mineralization for the Ligongling breccia pipe. This age suggests that breccia pipe formation and associated mineralization were coeval with the development of the Yangchuling porphyry W–Mo deposit.