ow is gold, from primary deposits from hydrothermal solution turn into a secondary deposit as alluvial placers

Answer 1

The first answer is absolutely correct. A vein of gold is exposed on a hillside. Erosion will eventually wash this gold downhill into a stream or river. Since gold is very ductile and the rock it is in inclusion with is brittle (usually quartz), the gold will seperate from the rock during the tumbling action of moving water and gravels and eventually "drop out" into a place where current flow no longer has the velocity to continue moving it downstream. This is a placer deposit.

The stream may reduce flow or change course leaving an alluvial bench where the gold remains until some prospector who recognizes this type of geologic formation and EUREKA!!

Answer 2

The original primary gold deposit is broken down over millions of years by weathering and water action. The gold, which is resistant to almost all types of chemical action remains in the form of grains and small nuggets. The action of fast flowing water in streams and rivers moves these small particles along its course. Gold, being very heavy drops to the bottom quickly when there is any reduction in the water speed. Placers are formed on the inside of bends in the water course or behind large obstructions where the water flow is slowed down a lot. Often the ancient river is long gone and the placers are left as deposits marking its old course