Известия Томского политехнического университета: Инжиниринг георесурсов (Dec 2021)
KHAAK-SAIR GOLD-SULFIDE-QUARTZ ORE OCCURRENCE (WESTERN TUVA): DATING, PT PARAMETERS, FLUID COMPOSITION, AND ISOTOPES OF S, O AND C
Abstract
The relevance of the research is caused by the need to determine the age and fluid regime of formation of Khaak-Sair gold-sulfide-quartz ore occurrence in listwanites, characterized by a peculiar ore mineral composition, expressed in the presence of mercurial gold, selenides (fichesserite, naumannite, timannite, claustallite) and tellurides (hessite, Te-bismuthite and coloradoite). The research is aimed to date and examine PT ore-bearing fluid parameters and geochemical peculiarities, and the fluid sources of Khaak-Sair gold-sulfide-quartz ore occurrence in Western Tuva. Methods. 40Ar/39Ar dating was carried out by the method of step heating. The optical studies of ores were performed on Olympus BX41 and P-213M optical microscopes. The mineral composition was detected using MIRA 3 LMU scanning electron microscope with EDU of Oxford Instruments Nanoanalysis Ltd. The temperatures, salt composition, salinities and fluid pressures were obtained from individual fluid inclusions using Linkam TMS-600 stage equipped with Olympus BX 51 optical microscope; the volatile composition of fluid inclusions was examined on Ramanor U-1000 spectrometer with the Horiba DU420E-OE-323 detector, Millennia Pro laser (Spectra-Physics); the bulk volatile composition of the fluid was determined on the Agilent 6890 gas chromatograph, the anions in the water extraction was estimated on the CVET-3000 ion chromatograph, the cation and trace elements were detected by ICP MS (Elan-6100); the sulfur isotope ratios in galena were calculated on Finnigan MAT Delta gas mass-spectrometer in double-entry mode (analysts V.N. Reutsky and M.N. Kolbasova, IGM SB RAS); δ18С and δ18О isotopic ratios in quartz and carbonates were examined on Stable Isotope Ratio Mass Spectrometer Finnigan™ MAT 253 with Finnigan GasBench II sampler and IAEA standards: NBS-18 and NBS-19 (analyst M.N. Pyryaev, IGM SB RAS) and Isoprime with AQS (Akita Quartz Standard, analysts H. Kavarai, O. Matsubaya, University of Akita), respectively. Results. The 40Ar/39Ar dating of synore listwanites is shown 379,4±4,4 million years that corresponds to the Late Devonian. We identified that the ore hosted listwanites were formed due to aqueous Na-K-chloride fluid with salinity of 3,4–6,5 wt. % NaCl eqv and temperatures at least 325–200 °C. Gold-sulfide-quartz veins were formed at P~0,5–0,75 kbar (~1,5–2,3 km) due to CO2-water chloride (Na-K±Fe) fluid containing CH4 with salinity ranged between 4,5 and 37,4 wt. % NaCl eqv. and temperatures from 320 up to 120 °C (I ore substage – 310–200 °С and II ore substage – 320–120 °С), and with fO2, fS2, fSe2 and fTe2 variations that have contributed to the diversity of Au, Ag and Hg mineral forms. The values of δ34S from galena vary from –0,6 to –0,4 ‰, and the calculated values of δ34SH2S of I ore substage fluid vary from +1,5 to +2,1 ‰ (for T=280–210 °C), and II ore substage fluid – +1,6...+2,6 ‰ (for T=290–190 °C), which indicates the magmatic origin of sulfur. Values of δ18О in quartz from ore veins vary from 17,0 to 17,4 ‰, dolomite – +17,4...+17,8 ‰, calcite – +16,5 ‰, and calculated values of δ18ОH2S of I ore substage fluid vary from +8,1 to +5,7 ‰ (for T=250–210 °C), and II ore substage – +6,7 ‰. ...–2,2 ‰ (for T=230–120 °C) suggested that in the early stages of the ore-forming process fluid was of magmatic origin, and in later stages it was mixed with meteoric waters. The values of δ13C in dolomite of I ore substage vary from –0,4 to –0,7 ‰; calcite of II ore substage – –0,3 ‰, and the calculated values of δ13C in the fluid vary from –1,2 to +0,1 ‰ (for T=250–210 °C) and from –3,3 to +0,5 ‰ (for T=230–120 °C), respectively. This presupposes carbon inflow from granitoid magmas and/or its borrowing from host rocks. The composition of the fluid was transformed from early to late substages from carbon dioxide to water chloride with a decrease in chloride amounts of alkali and alkali-earth metals (from 37,4 to 4 wt. % NaCl eqv).
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