Placer Deposits

Question 1

How are Placer Deposits Formed?

Answer 1

Mineral veins exposed at the earths surface are weathered. During weathering, the ore will be broken up by mechanical weathering or left as insoluble material by chemical weathering. The ore and gangue minerals will be seorated into individual grains.

The weathered material is then transported. Most placer deposits are concentrated by the action of moving water into rivers and the sea. During transport the sediment is sorted by grain size, hardness and density. Less resistant minerals will be worn away by the erosion process of abrasion and attrition or dissolved in water.

Question 2

What are the Requirements to Form Placer Deposits?

Answer 2

• Preexisting mineral veins exposed at the earths surface.
• Dense, physically and chemical resistant ore minerals.
• Erosion and transport processes.
• Suitable sites of deposition.

Question 3

What are the Types of Placer Deposits?

Answer 3

• Meander bends
• Confluence points
• Beaches
• Plunge pools
• Upstream projections: Dykes
• Upstream projections: Ripples

Question 4

What are the Properties Placer Deposit Minerals Have in Common?

Answer 4

• Hard with little or no cleavage… So they survive abrasion and attrition during transport.
• Chemically inert… So they are not dissolved and taken into solution.
• Dense… So they are deposited first when the current velocity slackens.

Question 5

What are the Advantages of Placer Deposits?

Answer 5

• Recent placer deposits are loose, unconsolidated sands that are easily accessible and cheap to mine.
• The ore minerals are easily separated from the gangue minerals when dredging and hydraulic mining so less waste is produced.
• Could be argued placer mining has less environmental impact than underground mining.

Question 6

What are the Disadvantages of Placer Deposits?

Answer 6

• Dredging and hydraulic mining stir up silt causing surface water pollution.
• Placer deposits tend to be small therefore they are quickly exhausted.