Noble-metal mineralization and conditions of formation of Au-Ag epithermal veins from Kyzik-Chadr Au-Mo-Cu porphyry deposit, Eastern Tuva

Research object. The results of mineralogical-geochemical, fluid inclusion and isotopic studies of Au-Ag epithermal veins of Kyzik-Chadr Au-Mo-Cu porphyry deposit (Eastern Tuva) are considered. The aim of the study is to examine mineralogical and geochemical peculiarities and conditions of formation of gold-sulfide-quartz veins from Kyzik-Chadr deposit to identify their ore-formation. Methods. The chemical composition of minerals was determined by SEM (Tescan MIRA 3 LMU with XMax 80 and INCA Wave 500 (Oxford Instruments Nanoanalysis Ltd). Fluid inclusion study in quartz were carried out using a Linkam TMS-600 thermostage with LinkSystem 32 DV-NC software and an Olympus BX51 optical microscope. The oxygen isotopic composition of quartz was determined on a FINNIGAN MAT 253 gas mass spectrometer. The isotopic composition of sulfur in sulfides was determined using a Finnigan MAT Delta gas mass spectrometer in the double-infusion mode. Results. A wide variety of Au-Ag minerals in gold-sulfide-quartz veins due to variations of fO₂, fS₂, fSe₂ and fTe₂ during ore formation was diagnosed: gold, Hg-gold, Hg-electrum, Hg-kustelite, weishanite (Au,Ag)_1.2Hg_0.8, sylvanite AgAuTe₂, petzite Ag₃AuTe₂, hessite Ag₂Te, schützite Ag₅Te₃, empressite AgTe, fischerite Ag₃AuSe₂, Se-uytenbogaardtite Ag₃AuS₂, acanthite Ag₂S, which are associated with altaiite PbTe, coloradoite HgTe, claustalite PbSe, fahlores of tennantite-tetrahedrite series and barite. The study of fluid inclusions (thermometry, Raman spectroscopy) in quartz and mineral thermometry (petzite–hessite–native Au paragenesis) determined that the ore veins were formed due to CO₂-water Na-K ± Mg-chloride fluid with salinity of 5.7–10.0 wt % NaCl eq with temperatures decreasing from 360 to 230°C and variations in fO₂, fS₂, fSe₂, and fTe₂. Oxygen isotopy in quartz indicates mixing of magmatic fluid with meteoric water (δ¹⁸O of fluid from +3.5 to +7.1‰). The δ³⁴S_H2S values of the fluid from +7.1 to +5.2‰ suggest that some sulfur was extracted from the host rocks. Conclusions. According to mineralogical and geochemical peculiarities and conditions of formation of gold-sulfide-quartz veins from Kyzik-Chadr Au-Mo-Cu porphyry deposit can be attributed to epithermal Au-Ag veins of intermediate sulfidation type, which are the product of a single porphyry-epithermal ore-magmatic system in the Kyzik-Chadr ore field.

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