Iron Formations

Iron oxides, sulphides, silicates and carbonates often form as chemical sediments on the sea floor. The iron minerals may be of sufficiently high grade and have good enough metallurgical quality to be suitable for steelmaking. The most useful are the oxide iron ores, magnetite and hematite, but iron carbonates may also be shipped as iron ore. Because the deposits are primary sedimentary beds, they are usually tabular in shape.

Intrusive Rocks and Mineral Deposits:
Intrusive rocks, particularly granite rocks, have a hand in forming many different kinds of deposits. Some of the deposits form in the intrusion themselves, while others form in the surrounding country rocks as a direct result of the intrusive activity. This is because with intrusion comes fracturing and hydrothermal activity.

Magmas can intrude into overlying rock gently or forcibly. If they intrude gently, they normally incorporate pieces of the country rock, which melt and become part of the magma. If they intrude forcibly, they can fracture or brecciate the country rock, creating spaces for the hydrothermal fluids to circulate.

As an intrusive rock cools, the major rock-forming silicate minerals – like feldspar – crystallite, and the remaining magma is left with the chemical constituents that have the lowest melting points. It contains large amounts of water, carbon dioxide and sulphur, and also has a lot of quartz. Intrusive rocks cool from the outside in, and this fluid-rich “late magma” is generally held inside the intrusion.

The “late magma” can be formed out of the magma chamber into fractures in the intrusion and in the country rock, cooling there and forming veins. Heat from the intrusion can also cause water to be released from the country rock itself, which may begin to circulate through fractures in the rocks and can carry minerals in solution, re-depositing them wherever it finds an open space. The circulating water can also move to porous country rocks such as sandstones or carbonates, leaving behind minerals that have come out of solution as the fluids lost heat or encountered different chemical solutions. And, as always, circulating hydrothermal fluids react with the rocks they pass through, causing alteration.