Formation of the Huxingshan tungsten deposit by pulsed magmatic-hydrothermal fluids: Insights from scheelite mineral assemblage, texture, and trace elements

Abstract

The Huxingshan tungsten deposit, located in the Jiangnan tungsten belt of South China, consists of scheelite-quartz-muscovite-calcite (SQMC) and scheelite-quartz-muscovite-fluorite (SQMF) vein-type mineralization hosted by the Yalan-Yuantang fault and the Niutitang Formation carbonate rocks. The textures and compositions of Sch-A (from SQMC veins) and Sch-B (from SQMF veins) were revealed by cathodoluminescence (CL) imaging and in situ LA-ICP-MS to decipher the nature and origin of the ore-forming fluids. CL images show that Sch-A is homogeneous, while Sch-B exhibits oscillatory zoning. The presence of quartz-muscovite assemblages in both vein types—common in greisen and vein-type W-Sn deposits—and the LREE-enriched patterns in Sch-A/Sch-B cores jointly indicate a magmatic-hydrothermal origin for the ore-forming fluids. However, Sch-A and Sch-B exhibit different REE patterns and yield two markedly distinct compositional clusters, with Sch-B having lower Sr, Nb, Mo, and higher Mn and REE contents than Sch-A. These mineral chemical differences, combined with the fact that fluorite exclusively occurs in Sch-B-bearing SQMF veins, suggest that Sch-A and Sch-B formed from two distinct magmatic-hydrothermal fluid pulses: a F-poor pulse and a F-rich pulse. The cores of Sch-A and Sch-B provide the earliest records of the two mineralizing fluid pulses, showing that (1) both fluid pulses initially exhibited LREE-enriched patterns, with the F-rich fluid pulse having lower LREE/HREE ratios than the F-poor fluid pulse, and (2) the F-poor fluid pulse is Sr-rich and REE-poor, while the F-rich fluid pulse is Sr-poor and REE-rich. The shift from LREE-enriched to MREE-enriched patterns from core to rim in Sch-A and Sch-B grains, accompanied by progressively decreasing REE and Sr contents, results from closed-system crystallization of scheelite during the two mineralization pulses. Sch-A and Sch-B have low Mo content (4.9–203 ppm) with variable Eu anomalies (0.56–2.89 for Sch-A and 0.87–4.21 for Sch-B), similar to those of scheelite from reduced skarn, indicating reduced conditions of the ore-forming fluids. Based on published geochronological data of the Huxingshan district, the overlapping Y/Ho ratios of both scheelite types indicate the mineralizing fluid pulses were likely sequentially released from a concealed granitic magma system emplaced at ca. 132.9 Ma. Our study at Huxingshan reveals that pulsed ore formation by multiple magmatic-hydrothermal fluid pulses is likely crucial for the formation of intrusion-related tungsten deposits.