Tracing the origin of ore-forming fluids in the Piaotang tungsten deposit, South China: Constraints from in-situ analyses of wolframite and individual fluid inclusion

Abstract

The Piaotang vein-type tungsten deposit, hosted by Cambrian metasedimentary rocks, is one of the largest vein-type hydrothermal deposits in South China. Wolframite is the dominant ore mineral and intergrown with quartz and cassiterite in the main mineralization stage. Wolframite, cassiterite and quartz crystals have δ18O values ranging from 2.5 to 6.8‰, 6.1 to 7.2‰, 10.3 to 13.7‰, respectively. δD values of fluid inclusions in these minerals are very homogenous and lie mostly between −66.1 and −78.3‰. Chondrite-normalized REE patterns of wolframite show HREE enrichment with significant negative Eu anomalies, which are different from those of hosting metasedimentary rocks. H-O isotopes of fluid inclusions and trace elements of wolframite indicated that the ore-forming fluids in the Piaotang tungsten deposit are dominantly magmatic in origin. Metamorphic and meteoric fluids were not involved in the main mineralized stage although the wolframite-quartz veins were hosted by metasedimentary rocks. Element pairs Zr/Hf and Y/Ho of wolframite are remarkably fractionated (Zr/Hf = 13.4–34.6, Y/Ho = 10.0–14.4). Almost all the analyses of wolframite display unusual tetrad effect REE patterns (TE1–3 > 1.1). These peculiar trace element characteristics of wolframites indicate that ore-forming fluids have high ligands F and/or Cl contents. LA-ICP-MS analyses of individual fluid inclusion show that ore-forming fluids contain all elements necessary for the formation of wolframite (W, Fe, Mn) and incompatible elements (e.g., Li, Rb, Cs). Cs/Na ratios are positively correlated with Rb/Na ratios. We conclude that the ore-forming fluids were reduced in nature and likely exsolved from the highly evolved granite that concealed in the vicinity of the deposit. Wolframite precipitation resulted from decreasing temperature and pressure of magmatic fluids during infilling along fracture, rather than fluid mixing and fluid-rock interaction.