Freibergite Series of Fahlore in the Mangazeyskoye Epithermal Silver–Polymetalic Deposit (West Verkhoyansk Region, Yakutia, Russia): Intergrowths, Heterogeneity, Chemical Composition, and Formation Conditions

Fahlore, which is the main Ag mineral of the Mangazeyskoye epithermal silver-polymetalic deposit (Yakutia, Russia) hosted in terrigenous strata, is studied in detail. It is established that the minerals of the fahlore group at the Mangazeyskoye deposit are dominated by kenoargentotetrahedrite–(Zn) and argentotetrahedrite–(Zn) at a subordinate amount of kenoargentotetrahedrite–(Fe), argentotetrahedrite–(Fe), and others. They are found in various mineral assemblages closely intergrown with each other, sulfides (galena, sphalerite, arsenopyrite, and boulangerite), and Ag minerals (diaphorite, owyheeite, pyrargyrite, stephanite, miargyrite, and freieslebenite). According to electron microprobe analysis, the Ag content of minerals of the freibergite series at the Mangazeyskoye deposit spans the entire range from 3 to 8 apfu, forming a solid solution with full almost continuous isomorphism between the monovalent Ag and Cu and between the divalent Fe and Zn. The freibergite series of fahlore is characterized by negative Ag–S correlation both at the deposit and in zoned heterogeneous aggregates. It is found that the formation of heterogeneity and oscillatory zoning in the fahlore aggregates of the Mangazeyskoye deposit is related to dissolution–reprecipitation reactions and exsolution of the fahlore solid solution. Four fahlore generations, which differ in chemical composition and associated minerals, are identified at the deposit. It is found that the composition of fahlore is related to the composition of the associated minerals: the maximum Ag content is typical of fahlore associated with pyrargyrite and/or miargyrite, fahlore with an Ag silver content in assemblage with chalcopyrite is decomposed to fahlore with a lower Ag content, and only fahlore–(Zn) is found in assemblage with sphalerite. It is shown that the ore is enriched in Ag from early to late mineral assemblages (both in terms of ore mineral composition and in chemical composition of fahlore) during retrograde dissolution–reprecipitation reactions. The formation temperatures of minerals of the freibergite series at the deposit, which are estimated using an Ag-fahlore geothermometer, mostly correspond to a range of 250–170°C and are consistent with those estimated by other methods. It is concluded that the fahlore assemblages were deposited from more alkaline fluids than the early quartz; the compositional changes of the fahlore were affected by the mineral-forming fluid, namely, its metal content and Cl and S activity. The compositional evolution of the mineral-forming fluid is related to its boiling, mixing, and dilution and a temperature change, which is accompanied by these events. Kenoargentotetrahedrite–(Zn) and kenoargentotetrahedrite–(Fe) of the Mangazeyskoye deposit are the Ag, Sb, Zn, and Fe end-members of S-deficient freibergite series and contain the maximum amounts of Ag and divalent metals ever found in nature.