Fluvial Placer Deposit (Gold)
Physical Process - Ground surface
Synonym
Alluvial placer deposit
Required Geological Setting
Neogene and Quaternary sediments
Occurrence
Fluvial placer deposits containing gold nuggets, Au-Ag alloy, where heavy minerals such as magnetite are selectively concentrated by water flows of rivers based on the difference of specific density. The gold fluvial placer deposits are at downstreams of gold-rich deposits such as hydrothermal Au-Ag deposits. The gold fluvial placer deposits are formed on the riverbeds at a gently flowing stream in fissures of riverbed slabs or at upstreams of obstacles such as boulders in water. As nugget grains are colligated in the course of flowing in the stream, the grain size of nuggets are bigger at downstreams and some nuggets cotain sand grains in them. As silver is more soluble in water than gold, the Au content of nuggets is bigger than electrum grains in the original gold deposits.
Mineral Assemblages
Magnetite, Gold