The composition and properties of ore-forming fluids are key to understanding the mechanisms of mineralization in ore deposits. These characteristics can be understood by studying fluid inclusions. Hydrates in fluid inclusions containing NaCl–H₂O and MgCl₂–H₂O were studied using cryogenic Raman spectroscopy. The intensity ratio of peaks at 3401, 3464, 3514, and 3090 cm⁻¹ shows a positive correlation with the concentration of hydrates in the inclusions, as does the ratio of the total integrated area of the MgCl₂ hydrate peak (3514 cm⁻¹) to the 3090 cm⁻¹ peak with the concentration of MgCl₂ (correlation coefficient >0.90). These correlations are important in the quantitative analysis of MgCl₂ in synthetic and natural NaCl–MgCl₂–CaCl₂–H₂O-bearing fluid inclusions. Semi-quantitative analysis of NaCl–MgCl₂–H₂O solutions indicates that peaks at 3437 and 3537 cm⁻¹ reflect the presence of NaCl in the solution. Further, a peak at 3514 cm⁻¹ is indicative of the presence of MgCl₂. The relative intensities of these peaks may be related to the relative abundances of NaCl and MgCl₂. A quantitative attempt was made on NaCl–MgCl₂–CaCl₂–H₂O system, but it was found that quantifying NaCl, MgCl₂ and CaCl₂ separately in NaCl–MgCl₂–CaCl₂–H₂O system by the secondary freezing method is difficult.

The Gacun deposit is a typical Volcanic Hosted Massive Sulfide (VHMS) associated with Late Triassic seafloor calc-alkaline felsic volcanics. Studies of zircon ages, petrology, major and trace element geochemistry, and Sr-Nd-Hf isotope geochemistry of volcanic rocks from the Northern Yidun arc were undertaken in this paper. We reshaped the Gacun magmatic system activity time, defined the origin of magma evolution, and proposed a metallogenic model of the deposit. Whole-rock major element compositions of the magmatic rocks in the Northern Yidun island arc indicate that they are a complete basalt–andesite–dacite–rhyolite assemblage, showing three obvious stages of composition evolution. They are enriched in large-ion lithophile and light rare earth elements, but depleted in high field-strength and heavy rare earth elements, with weak-to-negligible Eu anomalies (obvious in rhyolite). These geochemical features indicate that the Northern Yidun island arc is a magmatic arc based on ancient continental crust. The Ganzi–Litang oceanic subduction induced mantle melting and produced calc-alkaline basaltic magma, while the MASH processes at the bottom of the crust produced andesitic magma. Part of the andesite magma erupted to form andesite lava. The remaining part was mixed with magma produced via anatexis of ancient crust (approximately 20%–40% of the ancient crustal component), forming the ore-bearing rhyolite. Zircon U-Pb age data defines Gacun magmatic–hydrothermal mineralization sequence of events: At 238 Ma, arc magmatism led to the formation of andesite in the eastern part of the deposit. At 233 Ma, in the arc zone (the western part of Gacun deposit), a large-scale bimodal magmatism formed the main ore-bearing rock series of Gacun deposit, rhyolitic volcanic rocks. At 221 Ma, volcanic eruptions tended to end and sub-volcanic intrusion occurred, forming a lava dome, which was located under the ore-bearing rhyolitic volcanic rocks. The lava dome acted as a thermal engine and promoted hydrothermal circulation. The hydrothermal activity reached a peak at 217 ± 1 Ma, and the Gacun VHMS deposit was formed.