METAL AND MINERAL MICROINCLUSIONS IN ADAKITES FROM THE FRAMING OF THE UTANAK MASSIF (<i>Stanovoi superterrane, Russian Far East</i>) AS EVIDENCE FOR METAL ENRICHMENT IN ADAKITIC MAGMAS

Porphyritic amphibole–plagioclase andesites at the southern margin of the Utanak ultramafic massif (Stanovoi superterrane, Russian Far East) show low yttrium and ytterbium contents and high Sr/Y and La/Yb ratios typical of adakites. Microinclusions in these volcanic rocks are Cu–Ag–Au alloys, native silver, composites of gold and silver with Cu, Zn, In, Mn, Ni, and Mo minerals, as well as microparticles of platinum, silver sulfides and chlorides, pyrrothite, chalcopyrite, barite, tungsten oxide, crocoite, and cassiterite. The structural features of some microinclusions and their association with primary amphibole and plagioclase suggest their magmatic origin, while other microinclusions hosted in the quartz–chlorite–K-feldspar groundmass formed during later metasomatism. The Utanak adakites are characterized by elevated gold contents (up to 134 ppb) and, together with other manifestations of Mesozoic adakitic magmatism within the Stanovoi superterrane (Il’deus, Lucha, and Gabbrovyi massifs), are classified as ore–bearing (fertile) adakites according to geochemical criteria. We suggest that common association of adakitic magmatism with large porphyry, epithermal, and skarn deposits at convergent plate boundaries is related not only to the high water content and the oxidation state of adakitic magmas but also to the initial enrichment of the latter in various ore components.