Bauxite and Nickel Laterite Deposits and Supergene Enrichment

BAUXITE DEPOSIT

deposits refer to naturally occurring accumulations of bauxite ore, which is primarily composed of aluminium minerals such as boehmite, gibbsite, and diaspore, often mixed with other minerals like kaolinite and hematite.

These deposits are significant sources for aluminium production and are found in various geographical locations

1.Composition of Bauxite Deposits

: mostly formed of aluminum hydroxide mineral such as gibbsite, boehmite and diaspore and commonly contains impurities such as iron oxides and silica

2.Color of Bauxite Deposits

: Typically reddish-brown, due to iron oxides, can also be yellow, white, or gray, depending on impurity content

3.Texture and Structure of Bauxite Deposits

: Often pisolitic (rounded, pea-sized grains called pisolites) May also appear oolitic, earthy, or massive Can be layered in well-developed profiles

4.Physical Properties of Bauxite Deposits

-Soft to moderately hard

-Porous and lightweight

-Dull (earthy) luster

-Generally easy to mine due to near-surface occurrence

FORMATION OF BAUXITE DEPOSIT

Bauxite deposits generally originate from the process of weathering, most precisely chemical weathering. However different types of bauxite deposits have different modes of occurrence as well.

LATERITIC BAUXITE DEPOSIT

are residual (autochthonous) deposits formed in-situ through intense chemical weathering of aluminum-rich parent rocks, resulting in the concentration of aluminum hydroxide minerals within a well-developed laterite profile

blanket or layered deposit

-The most common formation of lateritic bauxite deposits

-It occurs as broad, laterally extensive sheets, follows the topography (plateaus, uplands), and its thickness varies but spreads widely

Lateritic bauxite typically occurs in

tropical to sub-tropical regions with high rainfall, warm temperatures, good drainages, and stable land surfaces (plateau, uplands)

Stage 1: Parent Rock Weathering

Stage2: Leaching of Soluble Elements (Laterization)

Stage 3: Residual Enrichment

Stage 4: Laterite Profile Development

MODE OF FORMATION of Bauxite deposits

Stage 1: Parent Rock Weathering Bauxite Deposits

-Aluminum-bearing minerals (e.g., feldspars) undergo chemical weathering

-Breakdown releases aluminum into the soil profile

Stage 2: Leaching of Soluble Elements (Laterization) Bauxite Deposits

-Rainwater percolates downward

-Removes: Silica, Alkalis (Na, K), Alkaline earth elements (Ca, Mg)

Stage 3: Residual Enrichment Bauxite Deposits

-Less mobile elements remain: Aluminum, Iron, Aluminum concentrates to form bauxite horizon

Stage 4: Laterite Profile Development Bauxite Deposits

-Continuous weathering produces a vertically zoned profile

-Distinct layers form from top to bottom

-Blanket-like, laterally extensive deposits, can cover large areas (plateaus)but its thickness varies (a few meters to tens of meters)

-Mineral dominance depends on climate and degree of weathering (Gibbsite, Boehmite, and Diaspore)

KEY CHARACTERISTICS of Bauxite Deposits

-Guinea - largest bauxite reserves globally

-Australia - largest producer of bauxite globally

-Brazil

-Surigao Del Norte

-Samar

-Leyte

LOCATIONS of Lateritic Bauxite Deposits

KARST BAUXITE DEPOSIT

deposits are accumulation-type (allochthonous) bauxite deposits formed when aluminum-rich materials, produced by weathering, are transported and deposited into karst features such as sinkholes, fissures, and cavities (acting as natural traps for bauxite materials) within carbonate rocks (limestone or dolomite).

sinkholes, fissures, and cavities

(acting as natural traps for bauxite materials) within carbonate rocks (limestone or dolomite)

-Stage 1: Source Formation

-Stage 2: Transport

-Stage 3: Accumulation in Karst

-Stage 4: Compaction and Enrichment

MODE OF FORMATION of Karst Bauxite Deposits

Stage 1: Source Formation Karst Bauxite Deposit

-Aluminum-rich rocks undergo intense chemical weathering

-Produces bauxitic material (rich in Al hydroxides)

Stage 2: Transport Karst Bauxite Deposit

Weathered materials are:

-Eroded

-Carried by surface water or gravity Movement is usually short-distance transport

Stage 3: Accumulation in Karst Karst Bauxite Deposit

Materials settle into:

-Sinkholes

-Cavities

-Depressions in limestone

These act as basins or traps

Stage 4: Compaction and Enrichment Karst Bauxite Deposit

Over time:

-Materials accumulate and compact

-Further leaching removes silica

-Aluminum becomes more concentrated

Karst deposits are generally irregular, lens or pocket like bodies, discontinuous and not wide spread unlike lateritic bauxite. Composition made up of boehmite, gibbsite and diaspore, however has often higher content of diaspore and boehmite than lateritic bauxite

KEY CHARACTERISTICS Karst Bauxite Deposit

China Greece Croatia Jamaica Hungary

LOCATIONS of karst bauxite deposit

"later" means "brick"

The name "laterite" is a derivative of the latin word

Nickel laterites

are thick, weathered layers (regolith) formed from the long, intense pervasive (chemical) weathering and leaching of underlying ultramafic rocks, especially serpentinites in tropical and sub-tropical climates (wet and warm).

where do nickel laterite deposits commonly occur along

ophiolite belts

1. Ultramafic Rocks

2. Tropical weathering begins

3. Leaching

4. Formation of new minerals

5. Wet-dry cycles (Redox processes)

6.Vertical movement of nickel

7. Mineralized Profile Zone

HOW NICKEL LATERITES FORM?

1.Topsoil (Red Limonite)

– The very top layer, rich in iron and organic material, giving it a reddish color.

2.Limonite (Yellow Limonite)

– Strongly weathered, iron-rich layer made mostly of clay and iron oxides.

3.Transition

– A thin, mixed layer between limonite and saprolite where minerals gradually change.

4.Saprolite (Weathered bedrock)

– Soft, moderate to strong weathered rock rich in nickel-bearing silicates like garnierite.

5.Bedrock (Fresh rock)

– The unweathered original ultramafic rock that serves as the source of the laterite profile.

Limonite Zone

-Uppermost Portion of a Laterite Profile

-Weathering: Strongly weathered
-Grain size: Clay and silt materials
-Colors: Brown to reddish-brown to yellow-orange
-Mineralogy:

• Iron oxides/hydroxides: Goethite (FeOOH), Limonite

• Manganese oxides: Asbolane, Lithiophorite

• Clay minerals: Kaolinite, Smectite

Saprolite Zone

Middle portion of the laterite profile

Weathering: Moderate to strong
Grain size: Silt, sand, and pebble- to boulder-sized weathered materials
Color: Orange-yellow and shades of green
Mineralogy:

• Nickel silicates: Garnierite (Ni-Mg silicate)

• Residual silicates: Serpentine, Talc, Chlorite

Bedrock

Lowest portion of the laterite profile

Weathering: Fresh to slightly weathered
Grain size: Boulders to massive
Color: Grayish-green to dark green
Mineralogy:

• Primary silicates: Pyroxene (dominant), Olivine

• Accessory minerals: Spinel, Chromite, Magnetite

Oxide (Limonitic) Nickel Laterite Ore

-This is the top layer of the laterite. It is rich in iron and holds nickel inside iron minerals.

-Found from limonite zone down to the upper saprolite. Nickel is mainly in goethite (an iron mineral).

-Typical grade: 1.0–1.6 % Ni.

-Makes up 60% of the world’s nickel laterite resources. Manganese oxides (e.g., asbolane, lithiophorite)are common and also carry Ni and Co.

-The very top (plasmic horizon + duricrust) has very low Ni.

-Nickel-rich zone starts just below, in the ferruginous saprolite.

-At the Mg-discontinuity, the chemistry suddenly changes (Mg goes up).

Location of Oxide Limonitic Nickel Laterite Ore

Location: East Pinares, Cuba.

Hydrous Mg Silicate (Saprolitic) Nickel Laterite Ore

-Found in the deeper layer of the laterite where nickel is stored inside magnesium silicate minerals

-Found in the middle to lower saprolite.

-Main minerals: serpentine, talc, chlorite, garnierite.

-Has the highest grades: 2–5% Ni.

-About 32% of world resources.

-Forms on serpentinized harzburgite peridotite.

-Needs good drainage, usually in uplifted or hilly areas.

-Regolith is usually <40 m thick due to erosion. Silicate ore layer: 10–15 m thick.

-Nickel can be very high (up to 40%) in veins, coatings, and boxworks.

Locations: New Caledonia, Indonesia, Pacific islands, Caribbean.

Location of Oxide (Limonitic) Nickel Laterite Ore

Clay Silicate Nickel Laterite

-This type forms where clay minerals trap nickel, usually between limonite and saprolite (transitional layer)

-Found from limonite to mid-saprolite (transition zone).

-Main minerals: Ni-rich clays (smectite, saponite).

-Grade: 1.0–1.5% Ni. Holds about 8% of global resources.

-Forms in flat, stable areas with low erosion.

-Usually found in thick saprolite zones (40–60 m).

-Formed over serpentinized peridotite.

-May contain secondary silica and magnesite.

-Nickel stays in the octahedral layer of smectite (clay structure).

Locations Western Australia (Murrin Murrin, Bulong) Southern Urals (Buruktal, Russia; Kempirsai, Kazakhstan) Burundi, Brazil Cuba, San Felipe

Location Clay Silicate Nickel Laterite

SUPERGENE ENRICHMENT

a geological process where weathering and descending groundwater redistribute and concentrate metals (most notably copper) into high-grade ore deposits.

SUPERGENE ENRICHMENT

Also called secondary enrichment. It happens over time, controlled by the water table, oxygen availability, and rock chemistry. It enriches an existing deposit rather than forming a completely new type of deposit from scratch.

supergene-enriched ore body

The deposit formed as a result of SUPERGENE ENRICHMENT is sometimes called a

1.Exposure and Weathering

2. Oxidation: The "Acid Engine"

3. Leaching

4.Transport

5. Precipitation: The Redox Boundary

HOW SUPERGENE ENRICHMENT WORKS

1.Oxidized Zone (Leached Zone)

2.Supergene Zone (Enriched Sulfide Zone)

3.Hypogene Zone (Protore)

THE THREE FUNDAMENTAL ZONES in Supergene Enrichment

-Tropical to arid climates (favorable for weathering)

-Areas with abundant sulfide minerals

-Regions with stable land surfaces (long weathering time)

-Porphyry copper systems (most common host)

SUPERGENE ENRICHMENT DEPOSITS ARE TYPICALLY FOUND IN

LOCATIONS OF SUPERGENE ENRICHMENT DEPOSITS

Chile Arizona, USA Zambia Philippines (Atlas)

-Pyrite- FeS

-Chalcopyrite- CuFeS₂

-Chrysocolla- Cu₄H₄Si₄O₁₀(OH)₈

-Bornite- Cu₅FeS₄

-Malachite- Cu₂CO₃(OH)₂

-Native Au

COMMON ORE/ MINERALS ASSOCIATED with SUPERGENE ENRICHMENT