Iron-rich sedimentary rocks

Iron-rich sedimentary rocks are sedimentary rocks which contain 15 wt.% or more iron. However, most sedimentary rocks contain iron in varying degrees. The majority of these rocks were deposited during specific geologic time periods: The Precambrian (3800 to 539 million years ago), the early Paleozoic (539 to 419 million years ago), and the middle to late Mesozoic (205 to 66 million years ago). Overall, they make up a very small portion of the total sedimentary record. Iron-rich sedimentary rocks have economic uses as iron ores. Iron deposits have been located on all major continents with the exception of Antarctica. They are a major source of iron and are mined for commercial use. The main iron ores are from the oxide group consisting of hematite, goethite, and magnetite. The carbonate siderite is also typically mined. A productive belt of iron formations is known as an iron range.

Classification The accepted classification scheme for iron-rich sedimentary rocks is to divide them into two sections: ironstones and iron formations

Ironstones

Ironstones consist of 15% iron or more in composition. This is necessary for the rock to even be considered an iron-rich sedimentary rock. Generally, they are from the Phanerozoic which means that they range in age from the present to 540 million years ago. They can contain iron minerals from the following groups: oxides, carbonates, and silicates. Some examples of minerals in iron-rich rocks containing oxides are limonite, hematite, and magnetite. An example of a mineral in iron-rich rock containing carbonates is siderite and an example of minerals in an iron-rich rock containing silicate is chamosite. They are often interbedded with limestones, shales, and fine-grained sandstones. They are typically nonbanded, however they can be very coarsely banded on occasion. They are hard and non-cherty. The components of the rock range in size from sand to mud, but do not contain a lot of silica. They are also more aluminous. They are not laminated and sometimes contain ooids. Ooids can be a distinct characteristic though they are not normally a main component of ironstones. Within ironstones, ooids are made up of iron silicates and/or iron oxides and sometimes occur in alternating laminae. They normally contain fossil debris and sometimes the fossils are partly or entirely replaced by iron minerals. A good example of this is pyritization. They are smaller in size and less likely to be deformed or metamorphosed than iron formations. The term iron ball is occasionally used to describe an ironstone nodule.

Iron formations

Iron formations must be at least 15% iron in composition, just like ironstones and all iron-rich sedimentary rocks. However, iron formations are mainly Precambrian in age which means that they are 4600 to 590 million years old. They are much older than ironstones. They tend to be cherty, though chert can not be used as a way to classify iron formations because it is a common component in many types of rocks. They are well banded and the banding can be anywhere from a few millimeters to tens of meters thick. The layers have very distinct banded successions that are made up of iron rich layers that alternate with layers of chert. Iron formations are often associates with dolomite, quartz-rich sandstone, and black shale. They sometimes grade locally into chert or dolomite. They can have many different textures that resemble limestone. Some of these textures are micritic, pelleted, intraclastic, peloidal, oolitic, pisolitic, and stromatolitic. In low-grade iron formations, there are different dominant minerals dependent on the different types of facies. The dominant minerals in the oxide facies are magnetite and hematite. The dominant minerals in the silicate facies are greenalite, minnesotaite, and glauconite. The dominant mineral in the carbonate facies is siderite. The dominant mineral in the sulfide facies is pyrite. Most iron formations are deformed or metamorphosed simply due to their incredibly old age, but they still retain their unique distinctive chemical composition; even at high metamorphic grades. The higher the grade, the more metamorphosed it is. Low grade rocks may only be compacted while high grade rocks often can not be identified. They often contain a mixture of banded iron formations and granular iron formations. Iron formations can be divided into subdivisions known as: banded iron formations (BIFs) and granular iron formations (GIFs). The above classification scheme is the most commonly used and accepted, though sometimes an older system is used which divides iron-rich sedimentary rocks into three categories: bog iron deposits, ironstones, and iron formations. A bog-iron deposit is iron that formed in a bog or swamp through the process of oxidation.

Banded iron formations vs. granular iron formations

Banded iron formations Banded iron formations (BIFs) were originally chemical muds and contain well developed thin lamination. They are able to have this lamination due to the lack of burrowers in the Precambrian. BIFs show regular alternating layers that are rich in iron and chert that range in thickness from a few millimeters to a few centimeters. The formation can continue uninterrupted for tens to hundreds of meters stratigraphically. These formations can contain sedimentary structures like cross-bedding, graded bedding, load casts, ripple marks, mud cracks, and erosion channels. In comparison to GIFs, BIFs contain a much larger spectrum of iron minerals, have more reduced facies, and are more abundant. BIFs are divided into type categories based on the characteristics related to the nature of their formation and unique physical and chemical properties. Some categories of banded iron formations are the Rapitan type, the Algoma type, and the Superior type.

Rapitan type Rapitan types are associated with the glaciogenic sequences of the Archean and Early Proterozoic. The type is distinctive as the hydrothermal-input has notably less influence on this formation's Rare Earth Element (REE) chemistry than other formations during this time period.

Algoma type Algoma types are small lenticular iron deposits that are associated with volcanic rocks and turbidites. Iron content in this class type rarely exceeds 1010 tons. They range in thickness from 10–100 meters. Deposition occurs in island arc/back arc basins and intracratonic rift zones.

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