Discrimination of two diverse fluid evolutions from the Nanyangtian scheelite deposit, southeastern Yunnan: Evidences from fluid inclusions and mineral geochemistry

Abstract

The Nanyangtian scheelite deposit is located in the Nanwenhe-Song Chay dome (NSCD), southeastern Yunnan. This deposit has undergone four metallogenic stages and is characterized by several kilometers of bedded scheelite-bearing skarn (NYT-II stage), feldspar-bearing quartz veins (NYT-III stage), and sulfides (NYT-IV stage) in the Neoproterozoic schist and gneiss, all of which exhibit similar fold deformations. Throughout the stages, T_h and salinity of fluids gradually decrease, weakly and positively correlating, representing a slow cooling process. NYT-I fluids (F-, H₂O-rich and high T, p), as indicated by the plagioclase within feldspar-bearing quartz veins, may have directly evolved from a highly fractional residual melt or a salt-rich aqueous melt, signifying the magmatic-hydrothermal transition. From NYT-II to NYT-III fluids, a wider variation of δ¹⁸O_H2O (−2.4 ∼ 5.1 ‰) to a narrow range of δ¹⁸O_H2O (2.8 ∼ 5.1 ‰), and a narrow δ³⁴S range (5.18 ‰ ∼ 8.62 ‰) also indicates that fluids may evolve from the relatively oxidized granitic magma. Furthermore, NYT-I fluids could extend throughout the entire fluid evolution, culminating in two diverse paths: one is a much purer magmatic water towards the NYT-III and NYT-IV fluids, and the other is a more meteoric water-dominated towards the NYT-II fluids. A lower water/rock ratio (W/R) existed in the NYT-II stage, leading to the formation of moderately oxidized Tungsten (W)-skarns and scheelites through the NYT-I salt-rich aqueous melts or their reactions with wall rocks in a stable environment. W/R ratios increased in the NYT-III stage, resulting in the formation of NYT-III feldspar-bearing quartz veins with CO₂ generation and alkalinity enhancement. As oxidation diminished, fluids gradually evolved into the NYT-IV fluids, forming sulfides.