the numerical thermodynamic model of ore-bearing PZ_2-3 –granitoids of vitim plateau

Vasiliev V.I., Khrustaljov V.K.

Geological Institute SB RAS, Ulan-Ude, Russia, vasil@gin.bscnet.ru, vkhrustalev@yandex.ru

P-T conditions of zones for orogenic situation were computed using author’s software (Vasiliev, Zhatnuev, 2007) in such a way as to the initial hypogene (trans-magmatic) solution has the temperature close to granite eutectic. The initial composition of ore-bearing trans-magmatic solution was calculated from the equilibrium with rare elements of unaltered enclosing effusive rocks (Ti, Mn, Cr, Ni, Co, V, Mo, Cu, Pb, Zn, Sn, Zr, Be, Y, Sr, Ba) and ore-bearing granitoids (Li, Rb, W, B, F, U) (Table 1).

Table 1. The calculated composition of ore-bearing solution Components Content, mol/kg of solution Li 0,018729290 Be 0,000035840 B 0,003607437 F 0,044740610 Ti 0,009695071 V 0,000129443 Cr 0,000072159 Mn 0,004635767 Ni 0,000037110 Co 0,000020192 Cu 0,000026595 Zn 0,000114268 Rb 0,002457066 Sr 0,000461843 Y 0,000042652 Zr 0,000368324 Mo 0,000001376 Sn 0,000003044 Ba 0,000604251 W 0,000027196 Pb 0,000013021 U 0,000043272 H2O 55,389069220

Table 2. The initial compositions of PZ1–granitoids and enclosing rocks, mass. % Oxides Granitoids PZ1 Effusive rocks Sediments SiO2 69,53 56,57 61,67 TiO2 0,36 0,93 0,93 Al2O3 14,95 15,33 16,23 Fe2O3 1,26 2,60 1,72 FeO 1,68 5,00 5,41 MnO 0,05 0,18 0,12 MgO 0,88 3,80 3,78 CaO 2,21 7,38 3,90 Na2O 3,74 2,50 1,06 K2O 4,47 1,97 2,31 H2O - 0,02 0,08 P2O5 0,15 0,19 0,16 SO3 - 0,04 0,25 Σ 99,28 96,51 97,63

The given elements together with the elements of oxides of the silicate analysis constitute the set of the model independent components. The averaged initial zones compositions, which are collected from three primary geological reports and a monograph (Khrustaljov, 1990), are shown at Table 2.

The quantity of model depended components was only limited by databases of «Selektor»: 109 condensed phases (b_Berman, s_Janaf, s_Sprons98, s_Yokokawa databases), 83 components of aqueous solution (a_Sprons98 database) and 12 components of gas phase (g_Reid database).

The calculation of model has shown the principal possibility of forming of chemical and mineral composition close to granitoids, with increased concentrations of rare elements as a result of solution evolution according to III scenario, and without such concentration – according to I scenario. The obtained data successfully correlate with the natural compositions of ore-bearing and non-ore-bearing PZ_2-3–granitoids (Table 3).

Thus, the advantage of the model consists in mathematically proven principal possibility of hydrothermal genesis of both ore-bearing and non-ore-bearing PZ_2-3–granitoids of Vitim plateau according to III and I scenarios correspondingly.

Undoubtedly, the imperfection of the conceptual chemical model for all scenarios is absence of carbon as an independent component; with taking carbon into account it was possible to model the genesis of carbonaceous shales which are spatially associated with granitoids. Unfortunately, necessary data relating to carbon contents are absent in geological reports and accessible literature.

Table 3. The comparison between averaged natural and calculated compositions of ore-bearing аnd

non-ore-bearing PZ_2-3 granitoids.

Note: the calculated compositions are reduced to 100%. Mean-root-square error for oxides of silicate analysis ± 0, 00743840, for rare elements ± 0, 00006188.

References

Khrustaljov V.K. (1990) Geochemistry and ore content of the Vitim plateau Paleozoic granitoids. Novosibirsk. Nauka. 135p. (in Russian)

Vasiliev V.I., Zhatnuev N.S. (2007) Realization of substance and heat distribution model in language SI ++ with use of PC “SELECTOR” // Geochemistry and ore formation of radioactive, precious and rare metals during endogenic and exogenic processes. Materials of All-Russian Conference. Vol.2 Ulan-Ude. Izd-vo of BRC SB RAS. P.119-121. (in Russian)