ELECTRICAL PROPERTIES OF GNEISSES IN THE UKRAINIAN SHIELD UNDER HIGH THERMOBARIC CONDITIONS

Abstract

Thermobaric variation of electrical characteristics of sets of different gneisses from the Central Ukrainian Precambrian Shield were studied. Experiments were carried out on dry, air-dried and fluid-saturated samples at different temperatures (T) (high temperatures included) and pressures (P). Analysis of mean values of the electrical parameters studied showed that pyroxene-biotite gneisses had the lowest electrical resistivity. On the other hand, their dielectic coefficient ε was highest. This regularity can be explained by pyroxene-biotite gneisses being greatly affected by secondary transformations. The result was serpentinization of plagioclase and development of chlorites and amphiboles among dark-coloured minerals. The above processes caused additional defect domains to appear in the crystalline lattices of minerals. The latter reduced the activation energy (E) of current-carriers and resistivity (ρ). It was established that ρ of gneisses decreases from the dry to the water-saturated state. That transition is especially significant in high-resistivity gneisses. As mineralisation increases from 0.1 to 100 g/1, ρ decreases by an average of ≈1.5 orders of magnitude. Given the same concentration range of the salts with which the solutions are saturated, the ε increase rate is greatest in the low-frequency range. A relatively clear-cut relationship between porosity and electrical properties was established for gneisses. It was shown that ρ of water-saturated gneisses decreases when porosity increases, while their ε increases simultaneously. Moreover, the difference between the absolute ε and ρ values of potentially gold-productive and unproductive strata of this region, established under atmospheric PT conditions, persists at high temperatures. Studies of the variation of electrical resistivity in dry, air-dried and water-saturated gneisses under high temperature and quasi-hydrostatic pressure, simultaneously applied, showed that it depended considerably on water content. The increase of quasi-hydrostatic pressure reduces E and the transition temperature from extrinsic to intrinsic conductivity significantly. On the basis of system analysis of experimental data obtained, as well as on the data of P and T distribution in the Earth's crust of the Central Ukrainian Shield and the probable depth of occurence of gneisses, electrical resistivity values were predicted down to 15 km depth.