World Placer Journal - 2001 - Volume 2, pages 65-72. | ||
These are valley floor placers and they support more than 10 placer gold mines, yet there is no sign any were deposited by water action! | ABSTRACT Gold placers are widespread in Mongolia. Most are assumed to be gold-bearing river alluvials. However this paper claims that many valley-floor placers lack any evidence of water-borne transport. Instead the placers have angular to subangular clasts that not imbricated and always poorly sorted; cross-bedding is absent; and fluvial channel geometry is lacking. The placers are re-interpreted as colluvial material that has probably moved downslope as water-laden slurries. The term 'solifluction placers' is suggested. The solifluction placers are confused with fluvial placers due to both types being prevalent on valley floors. The solifluction placers tend to have blurred contacts with the underlying bedrock and - in the Sergelen Goldfield - are the only, or dominant, sediment on almost all valley floors. Surface streams are absent or minor. Field evidence is from extensive pitting in the Sergelen Goldfield along transect lines across valley floors, and examination of 10 active placer gold mines. Solifluction placers are dominant in the Forest-Steppe Transition Zone and the Steppe Zone where extremely cold winters and semi-arid conditions favour solifluction and suppress surface water flow. The paper contrasts the solifluction placers with the fluvial-dominated alluvial placers of the Zaamar Goldfield deposited by the Tuul river. However solifluction placers occur with them, creating complex mixed sequences. | |
 SOLIFLUCTION GOLD PLACER... a typical solifluction placer cut open in the end wall of a placer gold mine in the Sergelen Goldfield. Most of the placer is contorted material with relic bedding, and there is no sign of alluvial stream deposition. | ||
 | THIN VENEER... | |
