## 4 Results
### Gold Alloys in the Ildeus Intrusion  
Figure 4 presents the statistical distribution of gold in the rocks of Ildeus intrusion based on the samples collected by the Khingan Minerals AS (personal communication, 2021) over the entire area of its relatively well-exposed central part (location of some samples listed in Table 1 is shown in Figure 1c). The general character of the distribution of gold contents in the ultramafic rocks of the Ildeus intrusion (Figure 4) suggests that at least a third of all of the samples is characterized by gold from grades in the range of 10 to 100 ppb, while approximately 10% of samples display gold grades in excess of 100 ppb, which is significantly higher than the gold dark values in ultramafic rocks (6-7 ppb) [59; 60] and average gold content of the Earth's crust (around 5 ppb) [61; 62; 63]. This significant enrichment of ultramafic rocks from the Ildeus intrusion in Au (Figure 4) suggests that gold in these cumulate ultramafic rocks may form its own mineral phases. Detailed SEM-EDA studies of Au-rich ultramafic samples from the Ildeus intrusion have established the ubiquitous presence of small (1-15 \(\upmu\)m) micro-inclusions of gold-, silver- and copper-bearing alloys in rock-forming minerals from these rocks (Figure 5). These microinclusions display predominant Cu-Ag-Au compositions in association with olivine, orthopyroxene and amphibie (Figure 5a,b). Some gold-bearing particles in orthopyroxorene are mostly composed of Au with Ag (up to 7 wt.%), Ni (up to 8.2 wt.%), Cu (up to 37 wt.%), and Zn (up to 8.8 wt.%) (Figure 5c,d). Silver grains with variable copper admixture (Figure 5e; Cu up to 7.7 wt.%) were observed in olivine as well as in the larger orthopyroxene grain intersected by very thin (1-3 \(\upmu\)m) chalcopyrite veinlets (Figure 5f). Numerous relatively large (50-200 \(\upmu\)m) particles of Au-bearing intermetallic compounds were recovered from olivine websterite sample IL-5 (most enriched in Au = 596.18 ppm) by rough crushing and gravitational separation, following the general methods described in [50]. These are irregularly shaped, partially facetted Cu-Ag-Au grains with multiple crystalline inclusions and specific inner structure (Figure 6). Figure 4: Statistical distribution of gold in the Ildeus intrusion. Figure 5: Small inclusions of copper-, silver- and gold-bearing alloys in rock-forming minerals from the ultramafic rocks of the Ideus intrusion. (**a,b**) Cu-Ag-Au alloys associated with olivine and orthopyroxene (**a**) and orthopyroxene and amphibole (**b**). (**c**) Ag-Au alloy included in orthopyroxene. (**d**) Ni-Zn-Cu-Au alloy included in orthopyroxene. (**e**) Cu-Ag alloy included in olivine. (**f**) Cu-Ag alloy included in orthopyroxene along with veinlet composed of chalcopyrite. Mineral abbreviations: Oll—olivine, Opx—orthopyroxene, Amp—amphibole, Ccp—chalcopyrite. Figure 6: General view (**a**) and polished cross-sections (**b–i**) of gold-bearing particles from the olivine websiterite IL-5. (**b**) Cu-Ag-Au(\(\pm\)Ni) alloys co-existing with native iron and orthopyroxene. (**c–e**) inclusions of orthopyroxene (**c**), plagioclase (**d**) and native tungsten (**e**) in Cu-Ag-Au(\(\pm\)Ni) alloys. (**f–i**) cross-section through a 180 \(\upmu\)m grain of Cu-Ag-Au alloy with distinct cellular (“honeycomb”) exsolution structure: (**f**)—high-contrast BSE image. Rectangle shows area scanned for the estimation of integrated bulk chemical composition. (**g–i**) individual distribution maps for copper (**g**), silver (**h**) and gold (**i**) based on their characteristic radiation. Scale in all microphotographs, where it is not specifically stated, is 20 \(\upmu\)m.  
Some Au-bearing particles are intergrown with native iron and orthopyroxene (Figure 6b) and occasionally contain micro-inclusions of orthopyroxene (-1-3 \(\upmu\)m; Figure 6c), Ca-Na plagioclase (0.5-2 \(\upmu\)m; Figure 6d) and native tungsten (less than 1 \(\upmu\)m; Figure 6e).