Mineralogy and geochemistry of tellurium in the some mining sites of Transbaikalia

. The first results of the study of the occurrence and mineral forms of tellurium in hypogenic and hypergenic conditions on the example of some deposits in Transbaikalia are presented in the article. Their paragenesis and the variations of tellurium content in ores and the hypergenesis zone are considered. Experiments on water leaching of tellurium, bismuth, lead, uranium and silver were carried out. The physicochemical model of mineral associations with the participation of these elements in conditions of the hypergenesis zone was calculated.


Introduction
Tellurium (Te) belongs to the poorly distributed chemical elements.Its Clark for different types of the Earth's crust rocks is 0.001 ppm [1] which is four times less than gold.Elevated concentrations of Te are noted in the ores of gold, polymetallic and some tungsten deposits [2].Relatively high concentrations of tellurium are in association with the noble metals Ag and Au in the form of tellurides and sulphosalts.According to [3] 101 hypogenic and 99 hypergenic tellurium minerals have been recorded.In most cases tellurium is associated with the noble metals and copper as well as with bismuth and lead which are close in the Periodic system of elements.
Tellurites and tellurates prevail among the total number of hypergenic tellurium minerals.Additional anions in tellurium minerals are represented by carbonates, sulfates, sulfites, chlorides, arsenates, hydroxides, and complex oxides.Single minerals contain aluminosilicate, vanadate, molybdate, chromate, and phosphate anionic residues.Many minerals contain crystall water.A significant number of minerals with mixed cationic and anionic composition has been diagnosed.The cations present lead, copper, iron, calcium, magnesium, bismuth, manganese, uranyl, titanium, nickel, aluminum, thallium [4].These data are consistent with the fact that thallium is sometimes fixed together with tellurium and selenium in the noncoherent tailings material of the abandoned Sherlovogorsky and Khapcheranginsky mining plants which developed tin-polymetallic deposits in southeastern Transbaikalia.The same applies to the peculiarities of the composition of the stored tailings of the Severo-Akatuevsky mine.
It is known, that during oxidation of hypogeneic tellurides of gold and silver (petzite, hessite, sylvanite, calaverite and others) tellurium is released during oxidation into tellurates and tellurites of lead and uranium, which are fixed as immobile new hypergenic phases.Many of these minerals are described in [5,6].

Materials and methods
The mineral and chemical composition of primary and oxidized ores of gold-silver, goldsulfide-quartz, tin-polymetallic deposits was studied using the optical and electron microscopy, and ICP MS in the laboratories of Geochemistry and Ore Genesis of INREC SB RAS, GIN SB RAS and SGS «Vostok Limited».We used 28 thin sections, 23 polished sections, and 32 samples for chemical analysis.Calculation of thermodynamic equilibria was carried out in PC "Selector" for multiphase system, given by the following quantities of components: SiO2 (quartz) -10, H2O -10, O2 -10, Pb -1, Ag -0.01, Au -0.001 mole.The accuracy of calculations was set at 10 -7 mole.
In the fahl ores of this deposit there was up to 0.015% of Te [7] found, where it associates with copper, lead, zinc, sometimes bismuth, mercury and manganese, replacing sulfur.Tellurium minerals are developed mainly in areas productive for gold and silver (figure 1).They are in association with pyrite, chalcopyrite, fahl ores, among which tetrahedrite prevails, usually containing along with the main chemical elements of its composition, copper, antimony and sulfur, iron, zinc and silver.Quite often, it is represented by the silver component of freybergite and argentotetrahedrite.Gold, sphalerite, vatanabeite (Cu4(As,Sb)2S5) (sample 9_14a), arsenopyrite, and lenaite (AgFeS2) are also in association with hessite.A complex sulfostibiotelluride of silver, gold, copper, and iron was found in sample 9_14d, containing (wt. %) (Ag 32.07 Au 19.02 Cu 9.95 Fe 2.33 Sb 9.91Te 17.94 S 10.57).
Of particular interest for this work are uraninite (figure 2) which was found in the Baley ore in association with quartz, pyrite, siderite, telluride, tetrahedrite, monazite, and arsenopyrite.Uraninite contains 3.34% Pb and up to 2.63% Th.In addition to uraninite small impurities of uranium are included in rhabdophane.Uranium is also contained in monazite in amounts less than 0.01%, which is fixed by X-ray spectra.Uranium in an amount of 3.07% was found in the closer undefined silver telluride, which has a composition of Ag 45.18 Te32.15Pb17.57U3.07 with a sum of 97.97%.
Taking into account the special behavior of lead in the hypergenesis zone in association with tellurium, attention is paid to the association of tellurides with galena and other minerals containing lead.Among these minerals robinsonite (Pb4Sb6S13) and bournonite (PbCuSbS3) [7] which were observed here earlier and madokite (Pb19(Sb,As)16S43), fiselite (Ag5Pb14Sb21S48), semseite (Pb9Sb8S21), gettardite (Pb(Sb,As)2S4), veenite (Pb2(Sb,As)2S5) in examined samples.The concentrations of silver, gold, tellurium and uranium in oxidized near-ore rocks from the contact of the 1 st ore zone of the Taseevsky deposit are given in (table 1).Tellurium is known in the ores of the Srednegolgotai deposit of gold-sulfide-quartz middepth formation associating with bismuth in tetradimite and other minerals.Here bismuth is associated with both bismuthite and tetradymite.It was found that the positive correlation between gold and bismuth grades is typical only for the sections of the deposit, where bismuth is in tetradymite containing tellurium, but where it is associated with bismuthite, it is not.The same is repeated in the oxidation zone.
In the ores of the Sherlovogorsky tin-polymetallic deposit, of the tellurium minerals only tetradimite has been established to date.Nevertheless, tellurium contents were determined in the loose material filling cavities in substantially quartz bodies with beryl, topaz, wolframite, bismuthite, and tetradimite (table 2).Tellurium was also found in the washed water used in the experiments on the extraction of chemical elements from loose sediments (table 3).The water used to wash the clay fraction had a pH of about 7.2.This indicates the possibility of tellurium migration not only in acidic but also almost neutral natural waters and its participation in the processes of mineral formation in association with lead, silver and uranium.The obtained analytical data showed the possibility of physical and chemical modeling of the formation of mineral phases with the participation of these chemical elements in hypergenesis.Due to the fact, that these same elements are also present in the tailings of tin-polymetallic and gold-bearing ores, the formation of hypergenic tellurium-bearing minerals is also possible in the geotechnogenic deposits formed in the tailings.Uranium minerals have long been known in the oxidation zone of the Sherlovogorsky deposit and are represented by monazite, metazinerite, uraninite, uranium-bearing tungstite and hydrotungstite [8], and with the exception of metazinerite have been studied very poorly.Nevertheless, U occurs in amounts exceeding hundredths of a percent at fairly intense migration (table 3).
Tellurium in the form of tetradimite was also found in the ore material of the Alexandrovskoe, Lubavinskoe and other gold deposits.
Thus, the mineralogical and geochemical peculiarity of tellurium prevalence in primary ores and the hypergenesis zone is its association with lead, silver, gold, and uranium.Since mobile forms of tellurium, lead, and uranium can be environmentally dangerous, it is relevant to consider the physical and chemical conditions of their transition into immobile forms.This is especially important for the understanding of the conditions of formation of hypergenic tellurium compounds with these elements and silver as an important mineral raw material.
Thermodynamic equilibria for the Si-Pb-U-Ag-Au-Te-O-H system were calculated using the PC "Selector".
For many possible secondary minerals of tellurium and uranium the changes of the standard Gibbs energies of their formation (ΔfG°) were calculated by the method [9] (table 4).The calculation results for variable amounts of tellurium and uranium are shown in (figure 3).Equilibrium with the solid phases aqueous solutions containing silver are characterized by increased alkalinity (pH > 11).The calculations show that primary gold and silver minerals with tellurium (petzite, etc.) concentrate gold during oxidation releasing tellurium into the composition of tellurates and tellurites of lead and uranium.

Table 1 .
Tellurium, gold, silver and uranium content in samples of oxidized near-ore rocks of the 1 st ore zone of the Taseevsky deposit, ppm.

Table 2 .
Contents of tellurium, silver, lead and uranium in the Sherlovaya Gora loose sediments.

Table 3 .
Tellurium, silver, lead, and uranium content in washed water from the Sherlovaya Gora loose sediments.