Известия Томского политехнического университета: Инжиниринг георесурсов (Mar 2020)
ON TRANSVERSE PETROGEOCHEMICAL ZONALITY OF MESOZOIC VOLCANITES IN THE ORE FIELDS OF THE TRANSBAIKALIAN SEGMENT OF THE MONGOLO-OKHOTSK FOLD BELT
Abstract
The relevance of the work is caused by the need to study the regularities of mineralization formation in the Transbaikalian segment of the Mongol–Okhotsk fold belt. The aim of the work is to establish the conditions of formation, to identify the presence of transverse geochemical zonality in the Mesozoic volcanic arc of the Transbaikalian segment of the Mongol–Okhotsk fold belt. Methods. The elemental composition of the rock was determined by the X-ray fluorescence method (GIN SB RAS, Ulan-Ude). Concentrations of rare-earth elements were determined by the method of sorption-atomic emission analysis with inductive-plasma (GIN SB RAS, Ulan-Ude). Silicate analysis was performed by the standard method of «wet chemistry» (GIN SB RAS, Ulan-Ude). Results. The author has established the transverse geochemical zonality of the Mesozoic volcanites of the Transbaikalian segment of the Mongol–Okhotsk folded belt, expressed in increasing concentrations from the frontal part of the volcanic arc of the Mongol–Okhotsk folded belt to its rear parts concentrations of K, Rb, Sr, Zr, Sb, Ba, Th, U, ce relations K/Na, Th/U, Rb/Sr, ∑TR. The spatial confinement of gold deposits to the frontal part of the Mesozoic volcanic arc, polymetallic and tin-polymetallic deposits to its rear parts is noted. Volcanites of polymetallic deposits are characterized by a higher degree of iron differentiation and a low degree of differentiation of magma chambers. Iron acid (a measure of oxidation states), the ratio to silicic acid rocks in the volcanites of gold ore deposits, varies from 0,19 to 0,90 (the average value is 0,53), in volcanites of polymetallic deposits – from 0,32 to 0,93 (average – 0,58). Such a pattern is typical for volcanics of island arcs of the continent-ocean transition zones. It was revealed that as the distance from the frontal part of the volcanic arc increases, the depths of magma generation of volcanic rocks of the Transbaikalian segment of the Mongol–Okhotsk folded belt increase. Such a pattern is characteristic to the Kuril-Kamchatka island arc. This is explained by increase in the depths of the magma generation zone with increasing distance from the frontal part of the subduction zone. The depths of magma generation in volcanic gold deposits, according to Rb-Sr ratios, are 15–20 km or more, and polymetallic and tin-polymetallic deposits are 20–30 km or more.
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