In-situ wolframite geochemistry and U-Pb geochronology of the Hongqiling W(−Sn) polymetallic deposit, southern Hunan (South China)

Abstract

The Hongqiling is the largest vein-type W(−Sn) polymetallic deposit in Hunan (South China), and is located in the world-class Nanling Range granite province. The orebodies display various mineralization styles, including quartz vein-type W-Sn, cassiterite-sulfide-type and quartz vein-type Pb-Zn mineralization. Here, we studied the ore veins from various depths at Hongqiling, and analyzed the different types of wolframite with scanning electron microscope (SEM) backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The results show that most wolframite samples are Fe-rich with varying Fe/(Fe + Mn) value (0.40–0.79), close to the wolframite end-member with uniform internal structure. The Tera-Wasserburg lower-intercept U-Pb ages of wolframite (160.0 ± 0.5 Ma (2σ; MSWD = 1.8) and 159.3 ± 1.7 Ma (2σ; MSWD = 1.2)) represent the timing of W mineralization at Hongqiling. The wolframite Nb-Ta contents are correlated positively with the contents of tetravalent (Ti, Zr, Sn, Hf) and trivalent (Sc, V, Y, REE) elements, indicating the substitution of elements with similar ionic radius is critical in the wolframite formation. Chemical composition of wolframite is controlled by both crystallochemical parameters and ore-forming fluid composition. The wolframite trace and REE compositions show obvious variation with depths and with increasing distance from the parent granite. The (Y + REE) vs. (Nb + Ta) has a strong positive correlation, and the chondrite-normalized patterns have distinct M−type tetrad effect (T 3 : Gd to Ho and T 4 : Er to Lu). All these characteristics indicate that the ore fluid was originated from the same highly evolved magmatic-hydrothermal system. Additionally, fluid inclusion (FI) microthermometry shows that the homogenization temperature of wolframite is higher (and wider range) than that of the coexisting quartz, but their salinity is similar. This implies that the wolframite precipitation occurred before quartz, probably caused by natural cooling. Integrating our new findings with published H-O isotope data, the quartz likely recorded fluid mixing, which caused the precipitation of later-stage quartz and sulfides. We suggest that the Hongqiling deposit was formed in the Late Jurassic, and was related genetically to the adjacent Qianlishan pluton. Spatial metallogenic temperature and geochemical variation of wolframite from the different orebodies at Hongqiling suggest that the quartz-vein-type W-Sn ores represent the mineralization system center, while the cassiterite-sulfide-type and quartz-vein-type Pb-Zn ores represent its distal part. We speculate certain W-Sn prospecting potential at depth beneath the Pb-Zn vein orebodies.

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