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Residual Deposits

The document discusses residual deposits such as bauxites and laterites. Bauxites form from weathering of aluminum-rich rocks under tropical conditions. They are typically blankets less than 30 meters thick. Laterites form from weathering of mafic and ultramafic rocks and can contain valuable metals like nickel, cobalt, chromium, and titanium. The document also describes supergene enrichment, where nearsurface chemical weathering mobilizes and reprecipitates metals to form enriched zones.

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yosia luther
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312 views3 pages

Residual Deposits

The document discusses residual deposits such as bauxites and laterites. Bauxites form from weathering of aluminum-rich rocks under tropical conditions. They are typically blankets less than 30 meters thick. Laterites form from weathering of mafic and ultramafic rocks and can contain valuable metals like nickel, cobalt, chromium, and titanium. The document also describes supergene enrichment, where nearsurface chemical weathering mobilizes and reprecipitates metals to form enriched zones.

Uploaded by

yosia luther
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Residual Deposits

Bauxites

Definition:

Why Bauxites and laterites?

Conditions necessary for an economic bauxite deposit:


Low Fe, Ti, alkalis, and alkali earths

Characteristics:
Irregular blankets several meters to tens of meters thick on top of their parent rock
(usually but not always).
Mineralogy and paragenetic sequence: Gibbsite (Al(OH)3) boehemite
(AlO(OH))+ diaspore (AlO(OH)).
Can be redeposited
Mined by open pit method.
Age: mostly post-Mesozoic
Main producers: Australia, Guinea, Jamaica, Brazil, India, Surinam and Balkan
Republics.

Types of deposits:
1-High level or upland bauxites
Directly on volcanic or plutonic rocks, no clay body in between.
< 30 m in thickness
In tropical and subtropical climates
Porous and friable, often with relict textures
Predominated by Gibbsite
Weathering controlled by structures in parent rocks
Examples: Ghana and Guinea
2- Low level peneplain bauxites
Somewhat transported, separated from their parent rock by kaolinitic underclay
~ 9 m thick
Along tropical coastlines
Pisolitic textures
Associated with detrital bauxites produced by fluvial and marine activity.
South America, Australia, and Malaysia.
3- Karst Bauxites
Oldest known
In Eastern Europe
On top of karst surfaces in limestone and dolomite
Structureless, earthy, concretions, . variable textures!
Predominated by Boehemite
Origin:
Conditions necessary for formation of bauxites:
1- favorable parent rock
2- porosity
3- high rainfall with intermittent dry spells
4- good drainage
5- tropical warm climate
6- low relief
7- long period of exposure
8- vegetation

Mode of formation:
1- Weathering
2- In situ leaching of elements and enrichment of residue in Al
3- Possible erosion and redeposition?
4- Addition of eolian dust.

B- Laterites

6 20 m thick on top of mafic and ultramafic rocks.


Nodular, red to yellow or brown hematite and goethite with as much as 20%
Al2O3.
Examples: Guinea, Guyana, Indonesia, Cuba and the Philippines.

C- Other Residual Deposits

1- Aurifeous laterites and bauxites


In greenstone belts.
Examples: Western Australia.

2- Ni and Co:
1-3 % Ni + Co in laterites on top of peridotites.
Ore mineral: Garnierite (Ni,Mg)3Si2O5(OH)4.
Example: New Caledonia.

3- Chromium:
3 % Cr in laterites on top of peridotites.
Ore mineral: In hematite and goethite! (used in steel).
Example: Western Australia.

4- Titanium:
12% Ti in soils (200 m thick) on top of alkalic igneous rocks.
Ore mineral: Anatase.
Example: Brazil (Parana Basin).
Supergene Enrichment

Definition

Conditions
1- Active chemical weathering with ground level lowered by erosion
2- Weathering under acidic (carbonated water) and oxidizing conditions
3- Time
4- Deep water table imposing reducing conditions
5- Restricted to non-glacial terranes.

Zones:
Oxidizing zone Gossan
Reducing zone below water table.
Possible drop in grade with depth.

Structural Control

Examples:
Step 1:
Pyrite + H2O + CO2 + O2 Fe(OH)3 + H2SO4 + H2CO3
Ccp + H2O + CO2 + O2 Fe(OH)3 + CuSO4 + H2SO4 + H2CO3

Step 2:
PbS Cv (CuS) + anglesite
FeS2 Cc (Cu2S)
Ccp Cv

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