PHYSICOCHEMICAL PARAMETERS AND AGE OF THE VASIL’KOVSKOE GOLD DEPOSIT (northern Kazakhstan)
M.O. Khomenko, N.A. Gibsher, A.A. Tomilenko, T.A. Bul’bak, M.A. Ryabukha, D.V. Semenova
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Gray and white quartz, gold, fluid inclusions, hydrocarbons, sulfur and carbon isotopes, Ar-Ar age
Abstract
A superlarge gold-ore stockwork of the Vasil’kovskoe deposit (with gold resources of more than 380 tons) is located at the contact of porphyroblastic granodiorites and diorites in northern Kazakhstan. The specifics of the Vasil’kovskoe deposit is a wide occurrence of gray (so-called ore) gold-bearing quartz, which, together with white quartz, composes quartz-sulfide veins and veinlets in the stockwork. Based on thermobarogeochemical and isotope-geochemical data, we have established that gray quartz and arsenopyrite of the deposit formed with the participation of K-Na-Mg-Cl-containing aqueous CO2-hydrocarbon fluids at 250-550 ºC, 0.1-2.5 kbar, and salinity of 7.0-22.5 wt.% NaCl equiv. (seldom, >30-40 wt.% NaCl equiv.). The cyclic recurrence of parameter fluctuations was accompanied by the deposition of gold, which led to the formation of gold-rich veinlet ores in the stockwork core. White quartz formed at lower temperatures, 120-310 ºC, and 0.2-1.0 kbar, with the participation of Ca-Na-Cl-containing fluids with salinity of 2.0-11.0 wt.% NaCl equiv. In addition to H2O and CO2, hydrocarbons and their derivates (paraffins, olefins, arenes, alcohols, ethers, aldehydes, ketones, and carboxylic acids) as well as nitrogen-containing (C2H3N, C3H9N, C7H5N, and C8H5NO2) and sulfur-containing (CS2, COS, SO2, C2H6S2, etc.) compounds (indicators of reducing conditions) were involved in the ore formation. The sulfur isotope composition of sulfides (δ34S = +5.7 to + 11.8‰) and the carbon isotope composition of CO2 in fluid inclusions in gray (δ13C = -2.1 to -4.6‰) and white (δ13C = -11.0 to -21.4‰) quartz as well as its positive and negative anomalies of Eu point to the crustal source of the fluids. The gray color of quartz is due to abundant CO2-hydrocarbon-containing inclusions, carbon particles, and sulfides. Crystallization of ore-hosting granodiorites proceeded in the period from 490 ± 4.4 to 443.5 ± 4.1 Ma. The age of the areal K-feldspathization of granodiorites, preceding the ore formation, is 375.2 ± 3.7 Ma. Formation of gold-including parageneses took place in the period from 311.7 ± 6.4 to 279.2 ± 2.5 Ma, i.e., lasted no less than 30 Myr.
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