On the Role of Endogenic and Exogenic Factors in the Formation of High-Grade Ores and Ore Shoots in the Vein Bodies of the Low-Sulfidation Tokur Gold Deposit (Amur Region)

Using the low-sulfidation Tokur gold–quartz deposit as an example, the long-debated problem of the genesis and relationships of primary and secondary ore shoots in low-sulfidation gold–quartz deposits (with sulfide contents of 0.5–2.5%) is considered as the basis for assessing their prospects and choosing a strategy for exploration or additional exploration of ore objects of this type of formation. Our research has established that the primary and oxidized ores of the vein bodies of this deposit are identical in grain size of native gold, primary mineral parageneses, spatial distribution of gold in vein bodies, and the composition of fluid inclusions. Thus, the rich gold of the oxidized ores of this deposit at subsurface levels was also formed at the hydrothermal stage. The low contents of dispersed gold in the sulfides of primary low-sulfidation ores (on average, 15–35g/t) and small amounts of sulfides in such orebodies make it virtually impossible for economically important secondary concentrations to form in the oxidation zone. Consequently, outcrops of gold-rich oxidized ores found on the surface of low-sulfidation deposits should be considered reliable indicators of the localization of primary ore shoots in them and, hence, the prospects of these vein bodies at depth. This should be taken into account when choosing directions for further deep exploration and additional exploration of ore objects that have been mined only at subsurface levels. Based on the examination of gold distribution in numerous eroded and “blind” vein bodies of this deposit, we have substantiated the spatial-genetic relation of the formation of primary gold ore shoots with areas of maximum openings of cavities, which at present are represented by swells of these bodies. The opening of the cavities led to a sharp decrease in fluid pressure, its supersaturation with gold, and a change in the gold transport forms. We believe that the gold was transported by fluids from the sites of a sharp pressure decrease to higher localization levels of ore shoots not only as soluble complexes, but also in solid-phase form according to the mechanism of its natural gas flotation, with the formation of nanonuclei of hydrophobic gold and their Auсr + gas bubble associates on evaporation barriers along the peripheries of released and growing gas bubbles during boiling, heterogenization, degassing, and supersaturation of the fluid with gold, gases and other components. The mechanism of effective solid-phase gold transport during ore formation was modeled in laboratory experiments.