Phase transformations of Bi, Sb, Se, Te and U in the oxidation model of wolframite rocks of the Sherlovaya Gora deposit (Eastern Transbaikalia)

. On the basis of the data of the chemical composition of wolframite rocks of the Sherlovaya Gora deposit, a model of their oxidation in an aqueous medium under standard conditions in the Selector PC is constructed. For Bi, Sb, Se, Te and U, equilibrium mineral phases and compounds are presented depending on the redox conditions of the "water-rock" systems.


Introduction
Sherlovaya Gora (Eastern Transbaikalia) is a unique geological site with deposits of semiprecious stones, tungsten, tin, bismuth and polymetallic ores (Fig. 1).The Sherlovogorsky ore district includes five deposits, of which Sherlovaya Gora is an intensely greisenised granite of the Sherlovogorsky massif of the Kukulbey intrusive complex (Fig. 2).The deposits were formed due to the functioning of the ore-magmatic system of the same name in the Jurassic-Cretaceous [Yurgenson and Kononov, 2014].Its absolute age covers the interval of 153-136 Ma.
The main structural elements of the ore field are the Sherlovogorsk granite massif of the Kukulbey magmatic complex, which is associated with the formation of rare-metal-tintungsten-volfram-bismuth mineralisation with semi-precious stones of the Sherlovaya Gora deposit, and bodies of subvolcanic acidic formations, which are associated with tinpolymetallic mineralisation of the Sopka Bolshaya and Vostochnaya Anomaly deposits.The general, general, stage of formation of the deposits is that in time, the greisen stage, which is associated with rare-metal-tin-tungsten-bismuth mineralisation with semi-precious stones, is replaced by a typical hydrothermal tin-polymetallic stage.
In connection with the greisenisation process, quartz-vein bodies containing beryl, topaz, layered silicates, and tourmaline along with quartz were formed.The ore component is represented by cassiterite, ferberite, ferruginous member of the isomorphic series ferberitewolframite-guebnerite.Bismuthite, nugget bismuthite, bismuthite, arsenopyrite, scorodite, ruzveltite, tungstenite, pharmacosiderite and other minerals are also present.To date, the total number of minerals identified within the ore field is more than 200 species [Kasatkin et al., 2014, Filenko et al., 2018;Jurgenson and Filenko, 2018].
In the forties of the last century, Sherlovaya Gora was considered as a potential uranium deposit, with reserves estimated at 147 tonnes, with a grade of 0.07% [Yurgenson and Kononov, 2014].Secondary uranium minerals have been identified in the hypergenesis zone of the deposit: The widespread occurrence of sulphide minerals in the ore district leads to their active oxidation in aeration zones with the formation of highly reactive solutions that interact with mining materials [Eremin et al., 2019].This leads to the formation of a large number of newly formed minerals [Rusal, 2015;Glukhova et al., 2018] (Fig. 3).

Materials and methods
Samples of wolframite rocks (Fig. 4) were collected on the western slope of Melekhinskaya sopka [Yurgenson and Kononov, 2014].The samples were crushed and dispersed into two size classes.They were chemically analysed by ICP-MS method in the SGS laboratory (Chita) and presented in Table 1.In the software package "Selector", based on minimisation of Gibbs energies of multicomponent multiphase systems [Chudnenko, 2010], models of oxidation of ore-bearing rocks at temperature were built 25 °C and atmospheric pressure.The ratio S:L(water)=1:10, chemical composition was set in the intervals of contents according to (Table 1), elements C, F, Cl, etc. according to [Yaroshevsky, 2006].Thermodynamic properties of substances and minerals were taken from reference books [Grenthe et

Results and discussion
From the chemical composition of the samples (Table 1) it can be seen that many elements exceed the clarks of the Earth crust: W, Bi, As, Sb, Y and heavy lanthanides, Sc, Zn, Cd, Cu, In, Pb, Be, Li, Rb, Cs, Nb, Ta, Sn, Mo, Zr, U, Ag, Au, Se, Te.There is a noticeable increase in the number of many elements in the class <0.1 mm.This may be related to the dimensionality of mineral phases and their hardness (brittleness).
Calculation of thermodynamic equilibria was carried out at variable oxygen content from absence to excessive amount of oxygen in the systems "solution-solid phase-gas".The amount of oxygen in the system determines the values of electrode potentials (Eh) equilibrium aqueous solutions.The solid phases associated with solutions show the behaviour of elements at redox geochemical barriers.Due to the large number of elements considered (Tab.1), the results of calculations are presented for Bi, Sb, Se, Te and U in (Tab.2).
It can be seen from Table 2 that under reducing conditions (Eh<-0.1 B) The elements in question are present in the native form (Bi), as well as in the form of antimonides, selenides and tellurides.At the same time uranium (IV) is a part of uraninite and coffinite.When the electrode potential changes from negative to positive values, bismuth transforms into carbonates and arsenates, antimony -into hydroxides and tungstibite, uranium -into uranyl compounds.Selenium is represented by native phase and selenite, tellurium -by tellurates.At high positive values of electrode potentials, the variety of mineral forms is represented mainly by classes of oxides and hydroxides.
To date, more than 300 uranium minerals have been reported [IMA, 2023], belonging to the chemical classes of oxides, hydroxides, peroxides, sulfates, phosphates, carbonates, arsenates, vanadates, molybdates, silicates, tellurites and tellurates, selenites, niobates, titanates, and one tungstate.For all mineral species, the standard Gibbs energies of their formation were calculated.Moreover, as can be seen from Table 2, synthetic phases are present at equilibrium: Li2UMoO10, CsVUO6, CsUO2OOH, minerals of such chemical compositions have not yet been found in natural conditions.
Tellurium (Te) belongs to chemical elements of low distribution in the Earth's crust [Yaroshevsky, 2006] and does not form its own deposits.Relatively high contents of tellurium are in associations with noble metals and copper in the form of tellurides and sulfosols.Te is easily oxidised by air oxygen and the oxides formed are well soluble in water, which favours its migration in hypergenesis [Missen et al., 2022].Tellurium according to the classification [Christy, 2015] is the most "mineral-rich" element with an extraordinary variety of crystal structures of secondary minerals [Krivovichev et al., 2020].According to [IMA, 2023], 102 hypogene and 99 hypergene minerals of tellurium are currently registered.We have determined elevated Te contents in the material of tailing dumps of the former of Sherlovogorsk MPP from 0.5 to 10 g/t (for 12 samples).Unlike uranium, for which compounds a large number of algorithms for calculating thermodynamic properties have been developed [Clark et al., 1998;Chen et al., 1999;Eremin et al., 2020], unfortunately, there is no such variety of methods for tellurium.For example, in [Mostafa et al., 1995], additive contributions of thermodynamic properties of substances are presented for 136 cations, 23 anions and 5 neutral ligands, however, for Te(IV), no data are available.About half of secondary tellurium minerals are represented by the chemical class of tellurites.On the basis of experimental data of [Majzlan et al., 2022] we have estimated Te(IV) contributions in the model extension [Mostafa et al., 1995] and used them in the calculation of standard Gibbs energies of tellurite formation.As can be seen from Table 2, tellurium in reducing conditions is in telluride phases, and in oxidising conditions in tellurates.Similar geochemical behaviour can be seen for selenium.For these two elements, thermodynamic properties have not been calculated for all possible minerals, especially for primary minerals.For example, there are 9 known mineral species in the Ag-Au-Te system.Thus, the modelling results for Te and Se can be considered preliminary and require additional studies.

Fig. 3 .
Fig. 3. Discolourations of newly formed salts on the sides of the Sherlovlgorsky MPP quarry lake.Our earlier experiments on sulfuric acid leaching of Sherlovogorsk greisen and tinpolymetallic ores[Epova and Eremin, 2018] showed the active removal of a large number of chemical elements in reaction solutions.The question of the fate of many of these elements in the components of natural landscapes, the conditions of formation and stability of their mineral phases remains open.Certain ideas about it can be given by models of calculations of thermodynamic equilibria in the systems "water-rock"[Eremin et al., 2018;Eremin et al., 2019].

Table 1 .
Chemical composition of crushed wolframite rocks of different size classes.The contents of macrocomponents Si-As, in %, other elements in g/t.Components (

Table 2 .
Equilibrium phases Bi, Sb, Se, Te and U in the model of oxidation of Sherlovogorsk rocks (Table1), the ratio of S:L=1:10, at varying oxygen content (O2) in the system.