## 2 Geological Background and Sampling
On the basis of the systematic drill holes, four concealed fine-grained granitic dikes, called 1#, II#, III# and IV# from the south to north, respectively, are identified within the Kunyushan monzogranite (Figure 2b). The former are usually fresh and non-deformed and have abrupt contact with the latter, where no baked margins, alteration or faults are observed [19]. These dikes are generally several kilometers long with thicknesses varying between 10 and 80 m (Figure 2b). They trend E-W with dip angles of 20-50\({}^{\circ}\) towards the south and are considered to be branches of the Sanfoshan granitic pluton to the west [19]. The gold orebodies are mainly concentrated in the central part of the fine-grained granitic dikes, except that a few are located in the upper or lower boundary between the fine-grained granitic dikes and the Kunyushan monzogranite (Figure 2c,d). They are generally vein-like and S-shaped lenses in shape, being tens to hundreds of meters long in strike with a maximum length of 366 m and several meters thick, varying between 0.91 and 2.85 m (average 1.17 m). These orebodies usually trend 270-285\({}^{\circ}\) and dip 20-28\({}^{\circ}\) (average 25\({}^{\circ}\)) SW, which generally  
Figure 2: Geological sketch maps of the Muping–Rushan gold belt (**a**) and the Xiawolong gold deposit (**b**). Adapted with permission from Ref. [20], 2022, Elsevier. Geological profile maps of prospecting line 27# (**c**) and 39# (**d**) of the Xiawolong gold deposit, showing sample locations analyzed in this paper. Note: the yellow-shaded curved surface indicated by the black curve shows the topography relief in Figure 2a. extend from +44 m level to \(-\)70 m level, with a maximum sloping depth of 200 m and a buried depth ranging from 46 to 132 m (Figure 2c,d). The proven gold reserve is about 2 t, with the Au grade varying from 1.5 to 52.6 g/t with an average of 11.47 g/t). The gold mineralization in the Xiawolong deposit is represented by the disseminated (Figure 3a) and stockwork-style (Figure 3b) ores occurring within the fine-grained granitic dikes, where no alteration and deformation occur (Figure 3a,b). Four typical gold-bearing ore samples for molybden Re-Os isotope analyses are collected from drilled cores, with the locations shown in Figure 2c,d. Samples XML-g4 and XML-g9 are disseminated-style ores with sulfides occurring as disseminations in the fine-grained granite (Figure 3a), whereas samples XML-g1 and XML1B2 are stockwork-style ores, where sulfides mainly occur in veinlets with only a few in disseminations; the quartz-sulfide veinlet shows a clear curvy boundary with the fine-grained granite (Figure 3b). The gangue minerals mainly include K-feldspar, plagioclase, quartz and biotite (Figure 3c,d), and the ore minerals are dominated by pyrite, pyrrhotite, magnetite, chalcopyrite, molybden, galena and minor native gold (Figure 3e-j). The molybden is usually intergrown with pyrite and galena, which occur along the grain margin of quartz (Figure 3k,l). The gold grains are mainly medium-fine-grained native, usually coexisting with pyrite, magnetite and molybden and occurring along the intersecting crystallographic planes of or as inclusions in the hosted minerals, such as pyrite and magnetite, with different shapes--droplets, angular or dendritic (Figure 3h-j)--indicating that they are precipitated simultaneously. Figure 3: (**a,b**) Photograph of representative disseminated and stockwork-style ore samples for molybden Re–Os dating, respectively. (**c,d**) Photomicrographs under transmitted light, showing the gangue minerals of K-feldspar, plagioclase, quartz and biotite. (**e-g**) Photomicrographs under reflected light, showing the ore minerals of pyrite, pyrrhotite, magnetite, chalcopyrite, molybden and galena. (**h–j**) Photomicrographs under reflected light, showing the medium-fine-grained native gold coexisting with pyrite, magnetite and molybden. (**k,l**) Photomicrographs under reflected light, showing molybdenite intergrown with pyrite and galena along the grain margin of quartz.