Geology Reference
In-Depth Information
were the exposed parts of a huge iron deposit, a discovery that led to the
establishment of the iron ore mines in the Pilbara. Now, some 50 years later,
Australia is known to have huge reserves of iron and is the world's major
exporter of iron ore.
Question: how should governments manage the development of mineral
resources? Should they guard known resources for future generations by
limiting the rate of exploitation and banning exports, or should they free the
market and encourage exploration in the hope of finding new deposits?
5.3.2 Types and Characteristics of Iron Deposits
A combination of two processes produces the richest iron ores. Fe minerals first
precipitate from seawater to form chemical sediment; then chemical weathering
upgrades the Fe content when the deposit is exposed at the surface. (The latter
process, called supergene enrichment, is described later in this chapter).
There are several different types of iron deposit, as listed in Table
5.4
. The type
known as “ironstone”, “minette” or “Lorraine type”, has been exploited throughout
Europe since the start of the iron age around 800 BC. Mining of the iron ores of the
Table 5.4 Types of sedimentary iron deposits
1.
Banded Iron Formations
Types
Algoman
Deposits of Archean age, in greenstone belts associated with volcanic rocks; Fe or probable
volcanic exhalative origin
Superior
Early Proterozoic age, on stable continental platforms; deposition of Fe from ocean water
at the time of increasing oxygen content of atmosphere and oceans
Rapitan
Neoproterozoic deposits associated with glaciogenic sediments
Facies
Oxide
The most important type - the Fe mineral is hematite or magnetite.
Unmodified sedimentary rocks contain up to 30-35% Fe
Carbonate
Alternating bands of chert and siderite (FeCO
3
)
Silicate
The Fe minerals are silicates such as greenalite (a serpentine or clay with the composition
(Fe
2+
,Fe
3+
)
2
-3Si
2
O
5
OH
4
), chamosite (phyllosilicate (Fe;Mg;Fe ) Al(Si Al)O (OH;O)),
glauconite ((K;Na)(Fe;Al;Mg) (Si;Al) O (OH))
Sulfide
Alternating layers of pyrite and shale rich in organic matter
2.
Ironstones (Minettes or Lorraine-type)
3.
Bog irons
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