## 5 Discussion
### Physico-Chemical Conditions of Cu-Ag-Au Alloys Formation  
The compositional data summarized above in Figure 7 indicate that solidification of Cu-Ag-Au alloys with compositional ranges of Cu, Ag and Au metallic components, observed in the grains that were analyzed in this study, occurs within the temperature range of 900-950 \({}^{\circ}\)C [64]. Textural and compositional features presented in Figures 6 and 7 suggest that nucleation and initial consolidation of these Au-bearing alloys into a separate mineral phase occurred at even higher temperatures that correspond to their liquid state during the crystallization of principal rock-forming minerals (e.g., olivine, pyroxene and plagioclase) within the Ildeus intrusion. This is consistent with the occurrence of olivine and orthopyroxene micro-inclusions in Cu-Ag-Au alloys (Figure 6c,d) and the general lamellar shape of many large Cu-Ag-Au grains, which is indicative of their solidification in the intergranular spaces between the rock-forming silicate minerals. Textural data indicate that the incorporation of microparticles of highly refractory native tungsten (Figure 6e) most probably occurred when the Cu-Ag-Au alloys were still in the liquid state. The preservation of the original crystalline shape and initial composition of native tungsten micro-inclusions (no alloying between W, Cu, Ag and Au has been observed) during transformation of Cu-Ag-Au alloys from liquid into solid state was possible due to the complete lack of physical and chemical interactions between crystalline tungsten and solid and liquid gold [66], silver [67] and copper [68]. Additionally, the observed meshy texture in some grains (Figure 6f, lower portion of the image) can be possibly explained either by shock deformation during crushing or by plastic deformations during high-temperature processes involved in the formation of these alloys. Another controlling factor, which can greatly influence the formation of Cu-Ag-Au alloys during the crustal fractionation of convergent zone magmas, is low oxygen fugacity. Although current petrologic models suggest that most arc magmas and their ultramafic cumulates are variably oxidized in comparison to mid-ocean ridge basalts (MORB) [69; 70], some recently published data indicate that modern and fossil subduction zones are characterized by substantial redox heterogeneity, with some portions of the subducted slab and mantle wedge associated with reduced conditions (negative AFMQ values) [36; 50; 71; 72]. Moreover, various geochemical proxies suggest that early fractionates of convergent zone magmas (primitive arc cumulates) can be generally reduced (similar to MORB) and increase in oxygen fugacity during their further crustal magmatic differentiation [73; 74]. Very low oxygen fugacity conditions during the formation of Cu-Ag-Au alloys in ultramafic samples from the Mesozoic Stanovoy subduction zone were reconstructed on the basis of the Cu-Ag-Au alloy co-existence with native iron (Figure 6b), as well as for a general consideration of the Cu-O phase diagram, which suggests that the Cu-Ag-Au alloys with chemical compositions observed in the Ildeus rocks are stable at the oxygen fugacity levels of Log(O\({}_{2}\)) \(<-4\)[65]. Previously, we found compositionally similar Cu-Ag-Au alloys in pyroclastic rocks from the Lesser Khingan iron district (Russian Far East), where they are present as micro-spherules indicating their solidification in a low density media (e.g., water or air) [65; 75] as opposed to the Cu-Ag-Au alloys in the Ildeus intrusion, which crystallized together with rock-forming and accessory minerals [36]. Oxygen fugacity estimates for Cu-Ag-Au alloys from the Lesser Khingan pyroclastic rocks are in the range of \(-2\) to \(-4\)[65]. The oxygen fugacity increase in the Cu-Ag-Au ternary system will immediately lead to the occurrence of copper oxide (cuprite) represented by the immiscible liquid or distinctive skeletal crystals [65].