УСЛОВИЯ ФОРМИРОВАНИЯ И ВОЗРАСТ РЕДКОМЕТАЛЬНОГО ОРУДЕНЕНИЯ КУКУЛЬБЕЙСКОГО РУДНОГО РАЙОНА (ВОСТОЧНОЕ ЗАБАЙКАЛЬЕ)

Abstract

The relevance of the research. Identification of conditions, sources, movements and sequence of formation of rare metal concentrations in different geological settings is primary and fundamental issues of ore geology and geochemistry. To come up with this one the authors have determined the formation conditions of rare-metal mineralization by modern methods and, furthermore, carried out isotope-geochronological investigations, based on the study of the typical deposits of the Kukulbey ore region. The main aim is to obtain new data on physico-chemical conditions for formation of rare metal mineralization, and determine the age of mineralization. Objects of the research are Bukuka, Belukha and Antonovogora deposits. Methods. Fluid inclusions were studied in polished thin sections by cryo-thermometry and Raman spectroscopy. Cryo-thermometric studies were carried out in the THERM-600 microcooler from Linkam. The samples were successively cooled to –190 °C and heated until the substance of inclusions was completely homogenized. During the observation, the temperatures of the eutectic, melting of ice and homogenization were measured. Melting temperatures of ice for two-phase fluid inclusions and melting of salt crystals for three-phase allowed establishing the concentration of salts in terms of NaCl-equivalent. The composition of gas and mineral phases of the fluid inclusions was studied by Raman spectroscopy – the Ramanor U-1000 spectrometer and the Horiba DU420E-OE-323 detector from Jobin Yvon, the Millennia Pro laser from Spectra-Physics; Confocal Raman Microscope alpha 300R from WITec. 40Ar/39Ar method of isotope-geochronological dating by the method of stepwise heating was used to establish the absolute age of mineralization. Results. Ore mineral associations of these deposits were formed from chloride high-temperature fluids saturated with carbon dioxide. Ore-bearing fluids ubiquitously contain reduced gases. The dominant one is methane, hydrogen sulfide, nitrogen and hydrogen are subordinate gases. The authors have traced an evolution of ore-bearing fluid within the crystallization of individual mineral associations: decrease in temperature and concentration of the solutions and conversion of cationic composition from essentially sodium to essentially potassium. Moderate estimates of depths were obtained: 6–8 km for the Bukuka, 4,5–6 km for the Belukha and 3–3,5 km for the Antonovogora deposit. The age of rare metal mineralization is established at Antonovogora deposit – 146,7±1,7 Ma (muskovite from quartz-muskovite greisen with wolframite); at the Bukuka deposit – 135±2,8 Ma (sericite from quartz-wolframite veins); at the Belukha deposit – 155,6±1,8 Ma (sericite from quartz-wolframite veins). Conclusions. Rare-metal mineralization of the Kukulbey ore region, based on the Bukuka, Belukha and Antonovogora deposits, was formed from high-temperature hydrothermal solutions of magmatic origin saturated with reduced gases (methane, hydrogen sulphide, nitrogen and hydrogen). The crystallization depth varies from 8 to 3 km. According to isotope-geochronological studies the formation of tungsten mineralization within the Kukulbey ore region occurred in the Late Jurassic-early Cretaceous. Thus, rare-metal deposits of this area are probably associated with both the Kukulbey and the Shakhtama complexes.