Formation conditions of placers in the Tapsa-Kaakhem gold zone (Tuva) and their relation to primary sources

摘要

The formation conditions of placers within the Tapsa-Kaakhem gold zone (Tuva) have been determined, and the relation between primary and placer gold deposits has been studied. Polycyclicity in the valleys of the region resulted in the formation of gold placers of various morphologies with stratified producing reefs in loose sediments differing in composition and age: Oligocene-Lower Pliocene, Eopleistocene-Lower Pleistocene, Middle Pleistocene, and Upper Pleistocene. A complex relation between placers and their primary sources has been established. In general, the macrocomposition of gold in the producing reefs of different ages reflects the composition of gold from the primary sources. From older to younger producing reefs, gold fineness varies more widely, showing a trend toward smaller values. This suggests that undiscovered primary sources belonging to another association exist in the source area. The same is confirmed by a drastic change in the typomorphic features, grain size, and roundedness of gold in the longitudinal section of the Bai-Syut River valley. The peculiarities of gold from placers formed by the rewashing of glacial drift have been established. In the Kopto placer, nuggets are widespread (10 mm), which fully retained the shape inherent to vein gold. Apparently, the nuggets were transported to the placer with glacial drift in quartz gravel, which protected them from rounding. Quartz crumbled completely, and gold became free in situ under frost action. The presence of nuggets in the Kopto placer permits predicting a cluster distribution of gold in the known thin quartz veins.