Mineral Deposits Weathering and Placer deposits

Weathering

is the natural process that breaks down rocks, minerals, and other materials at or near the Earth’s surface.It involves the physical disintegration of rocks and the chemical changes of their minerals.

Weathering

- Breaks the rock

Erosion-

Loosens/Picks up

Transport

- Carries it away

Mechanical (Physical) Weathering

Breaks rocks into smaller pieces without changing their chemical composition.

Frost wedging

– Water enters cracks, freezes, expands, and breaks the rock.

Thermal expansion and Contraction

– Repeated heating and cooling causes rock to crack.

Exfoliation

– Outer layers of rock peel off due to pressure release.

Abrasion

– Rocks are worn down by friction from wind, water, or other rocks.

Chemical weathering

This involves the breakdown of rocks through chemical reactions, changing the rock’s composition.

Stage 1 – Alkaline:

Water removes softer elements, leaving harder minerals like iron, silicon, and aluminum.

Stage 2 – Acidic:

Acid allows iron and aluminum to move slightly, forming mineral deposits like bauxite and laterites.

1. Hydration

Water molecules attach to minerals.

2. Hydrolysis

Water reacts with minerals → forms new minerals.

3. Oxidation

Minerals react with oxygen.

4. Solution

Some minerals dissolve completely in water.

Biological Weathering

happens when living things (plants, humans, animals, or microorganisms) help break rocks. They do this in two ways: physically (breaking rocks apart) and chemically (changing minerals)

A. Plant Roots

B. Burrowing Animals

C. Microorganisms

D. Human Activities

Examples of Biological Weathering

PLACER DEPOSITS

A natural concentration of valuable minerals formed by mechanical (gravity) separation during weathering, erosion, and transport of rocks.

A type of secondary mineral deposit formed when valuable minerals are physically separated from their parent rock and concentrated by natural agents such as water, wind, or gravity.

-High-Density Minerals- heavier than common sediments

-Chemical Stability- resistant to weathering

-Mechanical Durability- withstand abrasion during transport

-Liberated- freed from parent rock

-Association with Black Sands- presence of heavy, darkcolored minerals like magnetite and ilmenite

KEY CHARACTERISTICS OF PLACER DEPOSITS

1.Exposure & Weathering

The bedrock is exposed at the surface as an outcrop and breaks down physically and chemically. Valuable resistate minerals (Gold, Tin, Diamonds) are "liberated" or freed from their original quartz veins or ore bodies as the surrounding softer rock turns into soil or clay

2.Erosion & Transport

Gravity, wind, or water carry minerals away from the source.

Lag Effect

light materials (sand) stay suspended and travel far, while heavy minerals "lag“ behind along the bottom

3.Sorting

Flowing water separates materials; heavy minerals settle, lighter ones are carried farther.

4.Deposition

When water loses velocity (kinetic energy), it drops the heavy load into Placer Traps. These include river bends (point bars), bedrock cracks (riffles), and the "dead water" behind large boulders.

5.Accumulation & Enrichment

Over thousands of years, these cycles repeat. Successive layers of heavy minerals build up in the same traps, eventually forming a concentrated, economically valuable placer deposit.

riverbeds, stream channels, and along coastlines

PLACER DEPOSITS ARE TYPICALLY FOUND IN

Paracale, Camarines Norte Jose Panganiban, Camarines Norte Malaysia Indonesia Withwatersrand, South Africa

Locations of placer deposits

-Native Metals ( Au,Pt, Cu)

-Cassiterite (SnO2)

-Magnetite ( Fe3O4 )

-Diamond

-Corundum

-Zircon

Ore/mineral Content placer deposits

1.Alluvial Placers

Formed in rivers/streams where flowing water concentrates heavy minerals like gold. The most common and has the highest economic value type of placer

Eluvial Placers

Found on hillslopes at the base of the source rock. Minerals move only slightly via gravity or wash. Great indicators of a nearby "Mother Lode."

6.Paleoplacers

Ancient deposits that have been buried and turned into solid rock (lithified) over millions of years.

5.Eolian Placers

Formed by wind in desert environments. Wind strips away light grains, leaving heavy minerals concentrated in dunes or wind traps. Less common and lower economic value.

4.Beach Placers

Concentrated by wave and tidal action. Waves wash away light sand, leaving "black sands" (Magnetite, Zircon) trapped on the shoreline.

3.Residual Placers

Remain in place above the source rock after weathering removes lighter materials

7.Glacial Placers

Deposits from glaciers; poorly sorted and usually low in economic value

PROCESS OF FORMATION OF PLACER DEPOSITS

1.Exposure & Weathering

2.Erosion & Transport

3.Sorting

4.Deposition

5.Accumulation & Enrichment