## 5 Discussion
### Geologic Conditions of the Formation of Cu-Ag-Au Alloys in Magmatic Systems  
The textural and compositional characteristics of Cu-Ag-Au alloys in ultramafic rocks of the Ildeus intrusion indicate that gold-bearing compounds in cumulate fractionated from primitive arc magmas differ in chemical composition from gold grains in typical upper-crustal epithermal and porphyry mineralized systems [22; 26; 27; 37; 38; 41; 42; 43; 44; 77; 78; 79]. These textural and compositional differences, as well as the association of magmatic gold alloys with high-temperature rock-forming silicate minerals and other metallic phases (such as iron in Figure 6b and tungsten in Figure 6e), also suggest drastically different evolutionary pathways for gold formation in the upper mantle and deeper crustal settings (close to the magma source and within deep crustal magmatic conduits) in comparison with the shallow porphyry, mesothermal and epithermal environments of gold transportation and deposition. The relatively rarity of the native gold with these specific \"magmatic\" compositions of Cu\(\sim_{30}\)Ag\(\sim_{10}\)Au\(\sim_{60}\) can be most probably explained by the instability of this Au-bearing alloy under oxidized conditions of mesothermal and epithermal hydrotherms, which typically results in the almost instantaneous removal of the copper component. Consequently, the search for \"primary magmatic\" gold should concentrate on mineralized systems hosted in deep-seated, relatively reduced crustal and mantle rocks, which were not previously subjected to the upper crustal oxidation. Primary magmatic gold particles (clustering around Cu\({}_{30}\)Ag\({}_{10}\)Au\({}_{60}\) compositions) were documented in pyroclastic rocks associated with iron mineralization in the Lesser Khingan Range [65], mantle wedge peridotite xenoliths from Kamchatka [50] and adakites from the Stanovoy suture zone [54]. [PERSON] and [PERSON] [80] reported native gold particles with 11 wt.% copper in the hydrothermal Ni-Co arsenide ones from the chromitites in the serpentinized Beni Bousera Ultramafic Massif (Morocco). Gold and other noble metal enrichments were previously reported for both sub-arc mantle sources [19; 50; 81; 82; 83] and subduction-related crustal magmatic conduits [19; 36], as well as primitive basaltic and andesitic magmas in volcanic arcs [54; 84; 85]. The distribution of gold in differentiated arc-related plutonic and volcanic suites suggests that Au behaves as an incompatible element during the fractionation of primitive convergent zone magmas [36; 54; 85; 86]. This results in the accumulation of precious metals, especially gold, in evolved, late-stage intermediate to felsic residual liquids and contemporaneous enrichment of residual melts in subduction zones in sulfur and magmatic sulfides [20; 87; 88; 28]. This is consistent with the ubiquitous presence of late-stage magmatic barite in evolved pyroxites from the Ildeus intrusion [36] as well as adakitic dikes associated with Triassic plutonic complexes in the central segment of the Stanovoy suture zone [56]. In some cases, sulfur is present in convergent zone magmas during various stages of their evolution beneath arc volcanoes [89; 90], while on other occasions, sulfides are formed in differentiating arc melts through the assimilation of graphite-bearing crustal rocks and reduction in the initially oxidized arc basalt magma [87]. Under all these petrologic scenarios, gold is accumulated in differentiated arc basalts and late-stage residual magmatic liquids in subduction zone environments to the point of metal saturation during the advanced fractionation of convergent zone magmas, which may, in turn, trigger the formation of gold-rich globules and micro-nuggets [20; 65; 75] as well as the crystallization of gold-bearing intermetallic compounds [36; 54] during late-stage magmatic differentiation processes in the upper island-arc crust.