FLUORINE IN GRANITE FLUIDS BY EXPERIMENTAL ESTIMATIONS

Aksyuk A.M., Zaraisky G.P.

Institute of Experimental Mineralogy RAS, Chernogolovka, Moscow region, Russia, aksyuk@iem.ac.ru

Granites are formed in various geological-tectonic setting at active participation of the water fluid containing other active components: CO₂, Cl, F, etc. Fluorine is one of the main parameter of formation of rare metal granite, pegmatite, and related deposits because it influence on melting of rocks and take part in carry and adjournment of many ore elements. The role of fluorine in geological fluids still more not enough quantitative characterized. For quantitative estimations of fluorine concentration in granite fluids, the system experimentally calibrated mineral geofluorimeters has been developed (Aksyuk, 2002). It includes biotite (Bt), muscovite (Ms), Li-mica (Li-mica), apatite (Ap), and topaz (Toz) ones. The geoflurimeters take into account complex structure of natural minerals – solid-solutions and various affinity to fluorine of their principal minals. They allow on structure of a mineral with complex anionic and cationic isomorphism and P-T parameters of mineral formation to calculate values of fluorine concentration in the fluids, being in equilibrium with these minerals. The biotite- and Li-micas geofluorimeters like:

log M_HF (Bt,Li-mica) =log (X_F /(1-X_F))_Bt-1722/T(K)-1,107*X_Mg,Li+0,216(Al-2)+0,8958+log a_H2O,

where Т(К) is temperature on Calvin; М_HF is concentration of neutral particle HF^o in a fluid, equilibrium with mica, in mol/dm³; X_F=F/4, X_Mg.,Li = (Mg+Li)/Σoct; F, Mg, Li, Al mean numbers of these chemical elements in crystallochemical formula of biotite, calculated on 44 negative electric charges; Σoct is the sum of octahedral sites in the formula a_H2O - activity of water in a fluid, taking into account complexity of a natural fluid. Others geofluorimeters have a similar kind:

logM_HF(Ms)=log(X_F/(1-X_F))_Ms-1722/T(K)-0.272(Li+Mg)+0.216(6-Si)+0,185(Fe+Si-6)+1.419+log a_H2O

log M_HF (Ap) = log (X_F /(1-X_F))_Ap - [3657-5,246 P (kbar)]/T(K) + 0.7 + log a_H2O.

log _MHF (Toz) = log (X_F /(1-X_F))_Toz - 2580/T(K) + 0.85 + log a_H2O.

Molar concentration, М_HF, will be close to total HF concentration in a solution at temperatures above 500^оС and pressure up to 5 kbar. At presence of alkalis in a solution it is necessary to take into account the contribution of their fluorides in total concentration. Molar concentration of the dissolved substance in the diluted solution to which the majority of natural fluids concerns, at increased Р and Т can be accepted, as a first approximation, equal: M_HF^T,P =m_HF * ρ_sol^{T, P}, where m_HF is mole concentration, mol/kg H₂O; ρsol^T,P is density of a solution, kg/dm³. Density of a solution it is possible to accept for the majority of natural fluids on water (HGK model). The analysis of a mode of fluorine with the help developed geofluorimeters has revealed three various trends in lg M_HF-T relation, characteristic for granites to which different types of deposits are connected (Fig. 1).

Rather low fluorine concentrations are typical for barren granites (Ural) and ones with which associated Cu-porphyry deposits (Aksug, Shahtoma, Santa Rita). With decrease of temperature, concentration of fluorine in fluids is appreciablly reduced. Rather high HF concentrations (up to 0.1 mol/dm³) are typical for rare metal (W-Mo-Sn) deposits such as Akchatau (East Kazakhstan), Spokoinoe (East Transbaikalia). There are HF concentration in fluids of three main granite phases, pegmatite, and greisens remain almost on one enough high level at decrease of temperature from 800^о up to 500^оС. And, at last, high HF concentration (up to 2 mol/dm³) are characteristic for the fluids related to Ta-Nb deposits such as Orlovka and Etyka (East Transbaikalia) and advanced there “apogranite”, and also for topaz contained granitoids and ongonites.

A level of fluorine concentration in fluids and Zr/Hf ratio in rare metal granite, describing their differentiation, closely correlate with each other, being the additional indicator such as deposits and their efficiency on ore (Fig. 2, Table 1).

Table 1. Relationship of the F- and Zr/Hf indicators and ores in granites

mHF(mol/kg H2O)	ToC	P (kbar)	Type of ore	Zr/Hf
0.001-0.005	550-600	1-1.5	1) barren granite 2) granite, Cu-porphyry	30-40
0.03-0.05	400-500	1-1.5	greisen, Mo-W,W-Sn
0.1-0.2	650-800	1-1.5	1) granite, Mo-W, W-Sn 2) granite - parent one for Li-F granite	20-30
1-2	600-700	1-1.5	Li-F granite, Ta-Nb	5-10

The work was supported by RFBR grants: 06-05-64980, 08-05-00835 and Sci. Scool-3763.2008.5