DISTRIBUTION OF NOBLE METALS IN OPHIOLITE ASSOCIATIONS OF THE ALTAI-SAYAN FOLDED AREA

We carried out a comparative analysis of the distribution of background contents of PGE and gold in rocks of ophiolite associations of the Altai-Sayan area (ophiolites of Kuznetsk Alatau, Gorny Altai, and West Sayan) and the Mid-Atlantic Ridge (MAR). The sections of ophiolites and modern oceanic lithosphere (ultrabasites–gabbros–dikes–effusions) have been studied. The contents of gold, PGE, and accompanying elements in the rocks were determined by the radiochemical neutron activation, atomic absorption, and X-ray fluorescence methods, and the contents of volatiles, by gas chromatography. Modern and ancient structures show similar PGE and gold patterns. The studied rocks have contents of low-melting PGE of the palladium association commensurate with those in primitive mantle and are drastically depleted in high-melting PGE of the iridium association. In PGE contents ultrabasites of all studied regions of the Altai-Sayan area are close to primitive mantle. Modern oceanic basalts (like N-MORB of the MAR) are most similar to magmatic ophiolite complexes of Gorny Altai and West Sayan. Island-arc ultrabasites of Kuznetsk Alatau are depleted in Ir and are enriched in Au. Both in the ancient ophiolite associations and in the modern oceanic lithosphere of the MAR, the water content increases with the contents of PGE of the iridium association. Fractionation of PGE and gold occurs both as a result of magmatic differentiation of matter and under the effect of fluids at postmagmatic stages.