Abstract
Late Mesozoic Upper Amga alkaline rocks are located in the Amga tectonic mélange zone within the Aldan-Stanovoy shield, Russia. The main phases of the Upper Amga complex consist of alkaline syenite intrusions as well as the lamprophyre dikes and sills. The mineralogical features indicate that the lamprophyre may belong to minette. Recent geological works revealed the linear and area-specific geochemical gold anomalies in association with late Mesozoic alkaline magmatism in the Upper Amga gold district. The age of the syenite intrusions is 131.4 ± 1.5 (2.9) Ma, and the lamprophyre sills and dikes were formed at 134.9 ± 1.6, 132.3 ± 1.5 and 117.7 ± 3.4 Ma. The mineralization is represented by the REE-Ti-Th, polymetallic (sulfide) and telluride-gold types in the quartz-calcite-chlorite alteration of the main magmatic phases of the alkaline rocks. The fluid inclusions study shows that the sulfides were formed from hydrothermal fluids of CO2-N2-NaCl-H2O composition at temperatures ranging from 390 to 340 °C, with 44–22.7 wt% NaCl-eq, whereas the epithermal telluride-gold mineralization originated from low-temperature CO2 ± N2-NaCl-H2O fluids, with the homogenization temperature of 230–210 °C and a of 9.2–3.3 wt% NaCl-eq.
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