PACROFI VI - Electronic Program

Fluid regime of metamorphism of Lower-Paleozoic series and fluid inclusions in quartz veins of the Sarmich gold deposit (S.Nuratau, Western Uzbekistan)

Janna Askerova

Institute of Geology and Geophysics, The Academy of Sciences of Uzbekistan, 49 Khodzhibaev Str.,Tashkent, 700041,Uzbekistan; E-mail: ingeo@rubin.gov.uz

The Sarmich gold-quartz deposit (Karatau ore-field) is located in the Zarafshan-Turkestan structural zone, Kyzylkum-Nuratau region. The host rocks are a Lower-Paleozoic black shale series, composed of erosion products of andesite-dacite volcanic rocks. Their structural-tectonic position as well as composition are similar to those of the gold-bearing Besapan suite of North Nuratau and Central Kyzylkum. Carboniferous-phengite-chlorite and Carboniferous biotite-chlorite schists are of the most interest because the gold mineralization is hosted by them. The P-T regime of metamorphism is associated with the phyllite and green-schist facies of andalusite-sillimanite type, which in its turn is associated with the tectonic-magmatic origin of Karatau.

Physical-chemical conditions and the fluid regime of metamorphism is determined by: 1) P-T conditions; 2) thermal-flow capacity; 3) tectonic setting; 4) origin of material composition.

Specific metamorphic reactions and Gibbs free energies have been calculated for real mineral compositions (determined by electron microprobe) in order to determine energy level and metamorphism quantitative characteristics of the muscovite-biotite-chlorite paragenesis of regional metamorphism:

0.766Chl+0.416Fng+0.280K =0.587Mu+0.250Bi+1.078Q+2.643H2O

and for andalusite-biotite-muscovite paragenesis of contact metamorphism:

0.281Mu+0.194Chl+0.622Q=0.197Bi+0.961And+0.090K +0.860H2O

Coefficients have been calculated by the inert elements, Mg-Fe-Al; a deficiency in K and H2O is compensated from fluid.

The fluid obtained as the result of the above reactions proves the endothermic character of these reactions. According to the equations and the experimental data, the fluid is composed mainly of water, volatile compounds and leached metal ions. The porous fluid compositions, determined by equilibria calculations and estimated from fluid inclusion data are comparable. The reactions show that the fluid contains water; potassium ions and silicon are leached from host-rocks. H2O, CO2, CH4 are the main volatile components which are preserved in fluid inclusions of metamorphogenic quartz veins. The roughly calculated H2O-to-CO2 wt. ratio (H2O/CO2=1.3) in the fluid is correlated with H2O/CO2 in the host rocks: for Tusun suite (O2-3) H2O/CO2=1.15-1.85, for Karakargin (O1-2) H2O/CO2=1.05-1.8, for Givachisay (C2-O1) H2O/CO2=0.45-2.3. H2O/CO2 is the major factor describing chemical potentials of H2O and CO2, these reactions being controlled by these potentials.

The mobile forms of Au and As are trapped in fluids. Due to increase in T (up to 250-300oC) most of the higher hydrocarbons disintegrated into CH4 and C, and as a result, transported Au and As became free and precipitated. This explains overlapping gold and arsenic geochemical halos.

The reactions we are considering are accompanied by absorption of heat; the newly formed minerals are stable under these P- T conditions. With the help of the muscovite-chlorite and biotite-garnet geothermometers it was determined that T=270-300oC, P=1108-1254 bar for regional and T=500oC, P=234 bar for contact metamorphism. The temperature conditions were favorable for migration of Si and Au, which gave rise to formation of metamorphogene gold quartz veins. P-T conditions (1kbar, 250-280oC) have been obtained by fluid inclusion thermometry.

Given the calculated Gibbs free energy it becomes obvious that the transportation of fluid was insignificant. With the increase in T the phengite+chlorite association is dehydratated and the muscovite+ biotite association is formed, which has higher crystallization energy and lower free energy. The fluid and leached elements migrated into low-temperature zones together with accumulated energy, some part of which was lost by the increase in T while the rest was consumed by metamorphic reactions. The insignificant decrease in the rock's volume (0.174cal/bar = 7.3cm3) and H2O liberation give rise to some sharp increase in energy, and accumulated energy is transported by fluid for short distances.

The metamorphism of the host rocks of the Sarmich deposit is associated with folding and fault zones. The schist, cleavage and fracture zones all play the role of drainage structures and the fluids were buffered by the rocks. The metamorphic concentrations of ore-elements are leached by solutions and transported to more permeable zones where they formed gold quartz veins and ore-bodies. The commercial gold ores in the deposit have a post-metamorphic, overlapped character.