Geochemical features of the compositions of the Mesozoic igneous rocks of the Khokhoi ore field (Upper Amga gold-mining district, Aldan shield, North Asian Craton)

The article considers features of geochemical composition of the Mesozoic igneous rocks of the Khokhoi ore field, spatially located in the Upper-Amga gold-mining district of the north-western part of the Aldan-Stanovoy shield, at the junction of two large structural units - the Olekma granite-greenstone region and the Aldan granulite-gneiss area, in the zone of meridional deep Amga fault. Magmatic formations are represented by the Northern Boskho, Western Boskho and Upper Khokhoi massifs of the Lebedinsky monzonite-syenite-granite complex, intruding rocks of the crystalline basement and terrigenous-carbonate complex of the Siberian platform cover, and by rare tephrite dikes. Due to the accessibility of the study area, little attention was focused on the mentioned igneous rocks until recently. The study showed that magmatic formations of the ore field are geochemically specialized in lithophile and siderophile elements, and they belong to derivatives of trachy-andesite and andesi-basalt magma. All studied rocks have sufficiently close spectra of rare-earth element distribution, in particular, the rocks are poor in heavy rare-earth elements relative to light rare-earth elements. Presence of low negative Eu-anomaly for the Northern Boskho and Upper Boskho massifs may indicate the fact that they were formed from residual differentiates during fraction crystallization of rock-forming minerals with the involvement of plagioclases. Low Eu/Eu* values also indicate the fact that these rocks were formed during the late stage of the area evolution. Negative niobium anomaly, identified for all igneous rocks of the ore field, indicates magma contamination with continental crust. Considering interpretation of rare-earth elements and their ratio, it was supposed that igneous rocks of the ore field were formed in intracontinental conditions. It was generally concluded that there was an intensive mantle-crust interaction throughout the magmatism evolution of the Khokhoi ore field, beginning from intrusion of tephrite dikes bodies to intrusion of syenite rocks. The presence of long-lived mantle source, as well as contamination with crust material, was responsible for heat and fluid inflow that supplied circulation of thermal spring in the system. Heat and fluids caused additional mobilization of ore material from surrounding rocks, with further formation of gold-sulfide mineralization close to tephrite dikes and the Northern Boskho massif.