The Berezitovoe gold-polymetallic deposit(Upper Amur region,Russia)：Structure,mineralogy and genetic aspects

The Berezitovoe deposit in the Sergachi volcano-plutonic and metallogenic belt preserves evidence for polymetallic mineralization of multiple stages.The steeply dipping gamet-tourmaline-muscovite-quartz metasomatites（with K-Ar ages of 132 ± 2.9 and 127 ± 4.4 Ma） carry two distinct stages of mineralization developed at different times：（1） polymetallic mineralization and（2） gold-quartz.The deposit is located within Paleozoic gneissose granitoids of the Pikansky complex（dated as 379 ± 1.1 Ma by zircon U-Pb method） intruded by early Cretaceous porphyry-like granites of the Haikta pluton（dated as137 ± 0.67 Ma by zircon U-Pb method） and late Cretaceous dikes of porphyrites,porphyries,and lamprophyres.Evidence suggests the action of late gold-bearing hydrothermal fluids on the early polymetallic ores and the selective mobilization of some elements from these lead to redeposition together with complex sulphosalts.

Chronology and Geochemistry of the Berezitovoe Polymetallic Gold Deposit in Eastern Siberia, Russia and its Geological Significance

The Berezitovoe deposit is a large-sized Au-Ag-Zn-Pb deposit in the east of the SelengaStanovoi superterrane, Russia. Au-Ag orebodies are hosted by tourmaline-garnet-quartz-muscovite metasomatic rocks; Zn-Pb orebodies are hosted by granodiorites, porphyritic granites and tourmalinegarnet-quartz-muscovite metasomatic rocks. These orebodies are surrounded by wall rocks dominated by the Tukuringra Complex granodiorites, porphyritic granites, and gneissic granodiorites. The alteration includes silicification and garnet, sericitization chloritization, carbonatization and kaollinization. LA-ICP-MS U-Pb zircon dating indicates that the gold mineralization can be divided into two stages in the Berezitovoe polymetallic gold deposit（at 363.5 ± 1.5 Ma, and133.4± 0.5）.Hornblende-plagioclase gneisses of the Mogocha Group in the study area underwent Paleoproterozoic metamorphism（at 1870 ± 7.8 and 2400 ± 13 Ma）, gneissic granodiorite of the Tukuringra Complex yields a late Paleozoic magmatic age（at 379.2 ± 1.1 Ma）,and subalkaline porphyritic granitoid of the Amudzhikan Complex yield late Mesozoic magmatic ages（133-139 and 150-163 Ma）. Granodiorites of the Tukuringra Complex in the study area have high concentrations of SiO_2（average of 60.9 wt%）, are aluminum-oversaturated（average A/CNK of 1.49）, are enriched in the large ion lithophile elements（e.g.,K, Rb, and Ba）, U, Th, and Pb, are depleted in high field strength elements（e.g., Ta, Nb, and Ti）, and have slightly negative Eu and no Ce anomalies in chondrite-normalized rare earth element diagrams.Fluid inclusions from quartz veins include three types： aqueous two-phase, CO2-bearing three-phase,and pure CO2. Aqueous two-phase inclusions homogenize at 167℃-249℃ and have salinities of 4.32%-9.47% NaCl equivalent, densities of 0.86-0.95 g/cm^3, and formed at depths of 0.52-0.94 km. In comparison, the CO2-bearing three-phase inclusions have homogenization temperatures of 265℃-346℃,salinities of 7.14%-11.57% NaCl equivalent, and total densities of 0.62-0.67 g/cm^3. The geochemical and zircon U-Pb data and the regional tectonic evolution of the study area, show that the Berezitovoe polymetallic gold deposit formed in an island arc or active continental margin setting, most probably related to late Paleozoic subduction of Okhotsk Ocean crust beneath the Siberian Plate.