Lode gold deposits are widespread in orogenic belts of various ages and are a valuable gold source, but their genesis remains debatable. The close relationship between native gold and quartz was considered a reason to search for acid-magmatic sources of heat and fluids (i.e., granite intrusions), while small gabbro bodies were often ignored. Six minor gold deposits associated with NE-strike faults were studied in the Khudolaz area of the South Urals (Tukan, Bilyan-Tau, Fazly-Tau, Muildy-Tamak, Alasiya-II and Isyanbet-I). It was established, for the first time, that all of the studied deposits are similar geologically but differ in mineralogical diversity of ore-bearing quartz veins, which is due to the different composition of host rocks and ore-bearing intrusions of the Khudolaz (325–329 Ma, U-Pb) and the Ulugurtau (321 ± 15 Ma, Sm-Nd) ultramafic-mafic complexes. Results of the geochemical study of quartz veins (ICP MS) and their fluid inclusions (microthermometry, gas chromatography) showed that native gold was mostly precipitated at temperatures of 230–330 °C from a low- to moderate-saline (8–12 wt.% NaCl-eq.) H₂O–CO₂–CH₄-bearing fluid, when weakly oxidized or near-neutral conditions, were replaced by reducing ones. No significant differences between barren milky white and ore-bearing brownish quartz veins were defined, which indicates their common formation settings and an impulse pattern of vein injection. The stable pattern of the fluid salinity, along with low hydrocarbon and N₂ contents, as well as a narrow range of δ¹⁸O values, indicate a prevailing magmatogenic source with a certain influence of host rocks but without the influence of meteoric waters. Based on the presented data, the studied deposits were attributed to the epizonal orogenic type. This study shows the formation of lode gold deposits is possible without the participation of granite massifs.

The paper presents new data on the trace-element (REE, Y, and Sr) composition of calcite from stockwork Au–Cu porphyry ores and host rocks of the Kutuevsky occurrence, South Urals. Trace elements were analyzed by LA–ICP–MS in calcite and by ICP-MS in intrusive rocks and ophicalcite. We established that REE amounts and patterns of the calcite significantly varied and depend on the ore mineralization location in the metasomatic aureole. The ores in chlorite–sericite–quartz metasomatites in the central part of the gabbro-diorite and diorite intrusion are dominated by calcite, whose REE amounts and patterns are comparable to those of the intrusive rocks. We suggest that REE in the crystallized calcite were borrowed by fluid mainly from the intrusive rocks. Calcite in the ores hosted by biotite–epidote–chlorite–sericite–quartz metasomatites in the eastern inner-contact zone of the intrusive massif contains abnormally low REE amounts. The most likely REE source for this calcite is assumed to be the carbonate cement of the ophicalcites. REE fractionation was largely controlled by fluid temperature. When the calcite crystallized, the mineral-forming temperature exceeded 250°C, and therefore, Eu²⁺ dominated in the fluid, and the regime was reducing.

Link for citation: Kuzhuget R.V., Ankusheva N.N., Kadyr-ool Ch.O., Redina A.A., Prokopyev I.R., Ponomarchuk A.V. Khaak-Sair gold-sulfide-quartz ore occurrence (Western Tuva): dating, PT parameters, fluid composition, and isotopes of S, O and C. Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2021, vol. 332, no. 12, рр. 148-163. In Rus. 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). Ссылка для цитирования: Золото-сульфидно-кварцевое рудопроявление Хаак-Саир (Западная Тува): возраст, PT-параметры, состав флюидов, изотопия S, O и С / Р.В. Кужугет, Н.Н. Анкушева, Ч.О. Кадыр-оол, А.А. Редина, И.Р. Прокопьев, А.В. Пономарчук // Известия Томского политехнического университета. Инжиниринг георесурсов. – 2021. – Т. 332. – № 12. – С. 148-163. Актуальность исследования обусловлена необходимостью определения возраста и условий образования золото-кварцевого рудопроявления Хаак-Саир в лиственитах, характеризующегося своеобразным минеральным составом руд, выраженным в наличии ртутистого золота, селенидов (фишессерита, науманнита, тиманнита, клаусталита) и теллуридов (гессита, теллуровисмутита и колорадоита). Цель: определение возраста, условий образования, геохимических особенностей рудоносного флюида и его источников, золото-кварцевого...

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.