Geology Reference
In-Depth Information
Lorraine basin and nearby deposits of coal (needed to smelt the iron ore) in northern
France, Belgium and Britain fuelled the industrial revolution. Germany's designs
on the Lorraine iron resources were one of the causes of the First World War.
Ironstones
from the Lorraine region were known as “minette” or “small ore”
because of their low iron content. The grade was typically around 30%, about
20-35% less than in “fer fort”, the richer ore mined in other countries. In addition,
the iron is present in silicate minerals that are difficult to refine, and are
accompanied by phosphorous and other elements that further complicates the
extraction processes. With the discovery of the vast ore deposits in Brazil and
Australia, mining of European ores has became uncompetitive and one after another
the mines in the Lorraine basin have closed, the last in the late 1990s.
Ironstones occur as lenticular beds commonly associated with organic-rich black
shales. They contain little or no chert and the iron is present in minerals such as hematite
(Fe
2
O
3
) goethite (FeO.OH) associated with carbonate (siderite, FeCO
3
) and silicates
such as greenalite (a serpentine or clay with the composition (Fe
2+
,Fe
3+
)
2
3
Si
2
O
5
OH
4
),
chamosite (a phyllosilicate with the composition (Fe
2+
,Mg,Fe
3+
) Al(Si Al)O (OH,O)),
and glauconite (K,Na)(Fe,Al,Mg)(Si,Al)(O,OH). The iron oxides and chamosite are
often oolitic. In addition to the European deposits, ironstones are known and mined in
central USAwhere they are known as Clinton-type ores. They were deposited two well-
defined age brackets, in the Ordovician-Silurian and again in the Jurassic.
Ironstones are thought to form when iron on the continents is subject to deep
weathering in a warm humid climate, conditions that lead to the development of
lateritic soils. The initial iron enrichment and the growth of small round structures,
pisolites, took place in highly oxidized surface layer in response to low-temperature
and chemical and biogenic processes. The lateritic soils were then transported into
shallow waters in deltas or estuaries where current and wave action sorted and
concentrated the iron minerals.
Banded Iron-Formations
are the dominant type of iron deposit. The term is used
for bedded chemical sediments comprising alternating layers of iron minerals,
usually oxides or hydroxides, and fine-grained quartz or chert (Fig.
5.6
). The
banding is manifested at different scales: centimeter-thick beds of iron minerals
and chert are internally divided into millimeter or sub-millimeter lamellae of the
same minerals. Detrital components such as clays, or grains of quartz and feldspar,
are usually rare. In the major iron formations of the Hamersley Basin, the continuity
of the bedding is remarkable: a single 2.5-cm-thick band has been traced over an
area of 50,000 km
2
and varves at a microscopic scale are continuous for 300 km.
Banded iron-formations (BIF) were deposited at three different time periods, all
in the Precambrian. The oldest is 3.5-2.7 Ga; the second and by far the most
important from 2.5 to 2.0 Ga; and the third, far less significant, from 1,000 to
500 Ma. The tectonic setting of deposition and the types of associated rocks is
different in each case, and this has given rise the following names for each type:
Algoman, Superior and Rapitan, respectively.
Algoman-type deposits are relatively small and are found in Archean greenstone
belts in association with volcanic rocks. Superior-type deposits are named after
Lake Superior in between Canada and USA where they were first mined and
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