The U-Pb SHRIMP age of zircons from diorites of the Tomino-Bereznyaki ore field (South Urals, Russia): Evolution of porphyry Cu-epithermal Au-Ag system

摘要

The Tomino-Bereznyaki ore field lies in the western part of the East Urals volcanic megazone (20-30 km southwest of Chelyabinsk). The commercial Tomino porphyry (Mo, Au)-Cu deposit is localized in the east of the field, within a small mesoabyssal intrusion of quartz-diorite composition. The epithermal Au-Ag Bereznyaki deposit is confined to subvolcanic dioritic porphyrites in the west of the field. The western and eastern parts of the ore field have a tectonic boundary. Granitoids belong to a single volcanoplutonic complex of K-Na-quartz-diorite composition. The U-Pb concordant age of zircons from the ore-bearing dioritic porphyrite of the Tomino and Bereznyaki deposits is 428 ± 3 Ma (MSWD = 0.9) and 427 ± 6 Ma (MSWD = 1.1), respectively. A Silurian absolute age has been established for the Urals porphyry Cu ore-magmatic system for the first time. The diorites and acid metasomatites of both deposits contain a unique three-mica assemblage (Mu, Pa, and Mu0.36Pa0.64). The metasomatized diorites are of similar isotope-petrogeochemical compositions; they have close total REE contents (24-52 ppm) and REE patterns. Their Zr-Hf, Nb-Ta, and La-Ce diagrams show similar trends. The obtained data indicate the close time of formation of the porphyry and epithermal deposits and their probable genetic unity. The vertical evolution of the porphyry Cu column from meso- and hypabyssal to subvolcanic level includes the isotope (Sr, S, and O) crust-mantle interaction. The deposits formed at different depths expose on the modern surface as a result of the block tectonic processes in the ore field.