Composition characteristics and formation conditions of a large copper nugget from the Taymet deposit (Gornaya Shoria, Russia)

The paper contains the study results of a large copper nugget from the Taymet deposit (Gornaya Shoria, Russia). The deposit is situated in Southern Siberia in a poorly bare terrain and is located among basalts, trachybasalts and pyroclastic deposits of the Lower Cambrian Ust-Anzas Formation, in which porous and fragmented areas are widespread. The nugget is kept in the Central Siberian Geological Museum (Institute of Geology and Mineralogy SB RAS, Novosibirsk, museum № I-2/30). With the weight of about 700 kg, its maximum dimensions are 0.25×0.65×1.4 m. The composition of two fragments of the nugget was studied using the JEOL JXA-8230 electron probe microanalyzer with wave spectrometers, TESCAN Mira 3 scanning electron microscope with Oxford X-Max 50 energy dispersive spectrometer and the Horiba Lb HR800 Raman spectrometer. By the obtained results, copper is evenly distributed with its average content of more than 99 wt. %. The impurity elements are Si, Fe, Mn, S, Hg, Sr, Ga and Ag. Among them, silver content has the maximum value of about 0.050 wt. %. The samples are revealed for different-size (5–200 µm, seldom up to 1 mm) microinclusions of euhedral crystals of quartz and datolite, xenomorphic segregations of calcite and potassium feldspar, as well as submicroscopic grains of mercuric silver. There are small cavities partially or completely filled with quartz and calcite, along with their subordinate cuprite, chrysocolla and malachite. The transfer of copper is assumed to be carried out during the infiltration of reduced hydrothermal solutions, which are very poor in sulfur, in the composition of complexes with ligands predominantly of hydrocarbons and carbon dioxide, followed by their decomposition and the formation of a copper nugget.