The Yubileinoe Porphyry Gold–Copper Deposit (Western Kazakhstan): Geological Position and Conditions of Formation

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In the Uralian Fold Belt, there are quite numerous and well-studied porphyry copper (±Mo) deposits corresponding to the traditional “diorite” (most) or “monzonite” (Talitsa, Verkhneuralskoe) models. Along with them, there are relatively small but gold-rich massifs of porphyric granitoids, including the large Yubileinoe porphyry Au–Cu deposit, which is located at the southernmost extremity of the Urals. In this study, two main types of regional hydrothermal–metasomatic alteration were distinguished based on applying quantitative petrography and areal multielement geochemical studies in the scale of the ore district: (1) an earlier synvolcanic secondary alteration of volcanics, similar to those observed in volcanic massive sulfide-bearing fields (albitization, propylitic, and listvenitic alteration) and (2) a later plutonogenic alteration of the porphyry type. The plutonogenic hydrothermal–metasomatic (HM) complex is represented by K-feldspathization, hornfels and skarn alteration at the progressive phase, and propylitization, sericitization, and beresitization at the regressive phase. They are caused by hydrothermal alteration in the apical part of the stock, composed of the mineralizing Frasnian granite porphyry complex that hosts the Yubileinoe gold deposit. A lateral series of geochemical zonality (from the periphery of volcanotectonic structures to their center) has been established for the volcanogenic stage of hydrothermal activity: CrNiCo → PbZnCuCrNi → AuAg (CrNi) → BaAuAg. A large positive anomaly of the lithochalcophilic type was found for the HM plutonogenic complex in the ore field of the Yubileinoe deposit. The concentric zonality of this anomaly is characterized by the development of Ag, W, Sn, Pb, As, and Sb halos at its periphery, and Au, Cu, Bi and Mo at its focus (“core”). The stable and radiogenic isotope geochemical data for most of the porphyry copper deposits of the Urals indicate the predominant mantle source of their rocks and ore matter. Their paleotectonic position is reconstructed as a mature stage of intraoceanic island arcs. Unlike many other porphyry objects in the Urals, the totality of geochemical, isotope–geochemical and geological features of the Yubileinoe deposit indicate the predominantly crustal magma source. According to these features, this deposit is closer to Andean-type porphyry deposits, and its position can be reconstructed as an active margin of the Mugodzhar microcontinent, i.e., a suprasubduction regime, transitional from a mature island arc to the marginal continental one. According to the complex of features, this deposit in the Urals is a close analogue of the porphyry gold deposits of the Maricunga Belt in Chile. The magmatic complexes from the Silurian (Wenlock) to the Devonian (Frasnian) age, which are parental to the porphyry gold–copper systems of the Urals, correspond to the early phase of the Wilson cycle. This phase is the most ore-productive with the formation of giant Cr and Fe–Ti–V deposits associated with ultramafic–mafic complexes. It is likely that the differentiation of mafic magmas in the large-volume chambers occurring in the lower part of the lithosphere causes the appearance (as an extreme member) of diorite melts with a noticeable enrichment in gold and copper.