Page 52 - Sedimentology and Stratigraphy

Sedimentary Ironstone 39

at least 15% iron are referred to as ironstones or iron 3.5.2 Formation of ironstones
formations in which the iron is in the form of oxides,
hydroxides, carbonate, sulphides or silicates (Simon- Iron-rich sedimentary rocks are varied in character,
son 2003). Iron-rich deposits can occur in all types of ranging from mudstones rich in pyrite formed under
depositional environment, and are known from some reducing, low-energy conditions to oolitic ironstones
of the oldest rocks in the world: most of the iron ore deposited in more energetic settings. Most are thought
mined today is from Precambrian rocks. to have originated in shallow marine or marginal
marine environments, but it is not always clear
whether the iron minerals found in the rocks are the
3.5.1 Iron minerals in sediments original minerals formed at the time of deposition, or
whether they are later diagenetic products. For exam-
Magnetite (Fe 3 O 4 ) is a black mineral which occurs as ple, the presence of ooids suggests agitated, and there-
an accessory mineral in igneous rocks and as detrital fore probably oxygenated shallow water, conditions
grains in sediments, but haematite (Fe 2 O 3 ) is the under which all iron minerals formed should be ox-
most common oxide, bright red to black in colour, ides or hydroxides. It is therefore likely that the iron
occurring as a weathering or alteration product in a silicates found in some shallow-marine ironstones
wide variety of sediments and sedimentary rocks. (e.g. ooids of chamosite) may be altered goethite. It
Goethite is an iron hydroxide (FeO.OH) that is wide- is generally thought that sedimentary ironstones form
spread in sediments as yellow-brown mineral, which under conditions of lowered sedimentation rate of
may be a primary deposit in sediments, or is a weath- carbonate or terrigenous clastic material. Siderite-
ering product of other iron-rich minerals, represent- rich mudstones are most commonly associated with
ing less oxidising conditions than haematite. Goethite deposition in freshwater reducing conditions, such as
forms as a precursor to haematite in desert environ- non-saline marshes: where sulphate ions are available
ments giving desert sands their yellowish colour. The from seawater then iron sulphide forms in preference
oxidation to haematite to give these sands the red to iron carbonate.
colour seen in some ancient desert deposits may be a
post-depositional process. Limonite (FeO.OH.nH 2 O) is
similar, a hydrated iron oxide that is amorphous. In
thin-section iron oxides are opaque: magnetite is 3.5.3 Banded Iron Formations
black and often euhedral whereas haematite occurs
in a variety of forms and is red in reflected light. Banded Iron Formations (BIFs) are an example of a
Goethite and limonite are yellow-brown in thin-sec- type of sedimentary rock for which there is no equiva-
tion and are anisotropic. lent forming today. All examples are from the Pre-
Pyrite (FeS) is a common iron sulphide mineral cambrian, and most are from the period 2.5 to 1.9 Ga,
that is found in igneous and metamorphic rocks as although there are some older examples as well (Tren-
brassy cubic crystals (‘fool’s gold’). It is also common dall 2002). As their name suggests, BIFs consist of
in sediments, but often occurs as finely dissemi- laminated or thin-bedded alternations of haematite-
nated particles that appear black, and may give a rich sediment and other material (Fig. 3.11), which is
dark coloration to sediments. In thin-section it is opa- typically siltstone or chert (3.3) (Fralick & Barrett
que and if the crystals are large enough the cubic 1995). Individual layers may be traced for kilometres
form may be seen. There are several silicate min- where exposure allows and units of BIF may be hun-
erals that are iron-rich: greenalite and chamo- dreds of metres thick and extend for hundreds of kilo-
site are phyllosilicate minerals (minerals with metres. The origin of BIFs is not fully understood, but
sheet-like layers in their crystal lattices) that are they probably formed on widespread shelves or shallow
found in ironstones and formed either as authigenic basins, with the iron originating in muddy deposits on
(2.3.2) or diagenetic (18.2) products. Glauconite the sea floor, possibly in association with microbial
(glaucony) is also a phyllosilicate formed authigeni- activity. The source of the iron is thought to be either
cally in shallow marine environments (2.3). The most hydrothermal or a weathering product, and could only
common iron carbonate, siderite, is considered in have been transported as dissolved iron if the ocean
section 3.1.1. waters were not oxygenated. This is one of a number