5.3.1. Microthermometry

Fluid inclusion microthermometry was performed on all qtz vein stages. However, the vast majority of inclusions were too small for complete microthermometric analysis, so only the homogenization temperature (Th) was measured [34]. All the examined samples show dense populations of fluid inclusion (cloudy area), so the FIA (fluid inclusion assemblages) could not be determined and it could not be distinguished whether the inclusions were primary or secondary. The size of inclusions ranges from 1 to 3 μm (mean value 1.5 ± 0.5 μm). Fluid inclusion microthermometry was performed on several fields of interest along the vein and focused on single inclusion with a moving bubble at room temperature up to 200–300 ◦C; at temperatures >350 ◦C, the moving bubble can no longer be seen. Most fluid inclusions show liquid and vapor phases with dark to colorless appearances. Inclusions have rounded-elongate shapes and sometimes occur as trails or solitary inclusions (Figure 9c).

**Figure 9.** (**<sup>a</sup>**,**b**). Homogenization temperatures and fluid compositions of stage 1–3 veins. (**c**) Photomicrograph of representative fluid inclusions (plane-polarized light).

Fluid inclusions in the qtz–mol veins homogenize at 142–340 ◦C (mean value, 267 ± 46◦C), and the Th of qtz–py veins span from 115 to 359 ◦C (mean value, 242 ± 65). The homogenization temperatures of fluid inclusions in the qtz veins are in range 108–295 ◦C (mean value, 194 ± 45) (Figure 9a and summary of microthermometric data is provided in Supplementary). The fluid inclusions exhibit a bimodal population, consisting of a low mean value of the qtz vein and a relatively higher value of the qtz-mol and qtz-py veins. This indicates that two different processes occurred in the Erdenet system. Later, we use the homogenization temperature (Th) value to estimate the minimum pressure condition of vein formation by using the isochore of Th and the Tm-ice of fluid inclusion in P-T space provided by [35,36].
