Mineral Deposits and Intrusive Rocks

Intrusive rocks, granitic rocks in particular, have a hand in forming a wide variety of deposits. Some of the deposits form in the intrusions themselves, while on the meantime others form in the surrouding country rocks as a direct result of the intrusive activity. This is for the reason that with intrusion comes fracturing and hydrothermal activity.

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

In the time that an intrusive rock cools, the most importat rock forming silicate minerals crystallize, as is the case of feldspar, and the left over magma is left with the chemical constituents that have the lowest degree of melting points. It contains very large quantities of water, sulphur and carbon dioxide; it also contains a large amout of quartz. Intrusive rocks cool from the outside to the inside, and this “late magma” which is rich i fluids, is for the most part held inside the intrusion.

The “late magma” can be forced out of the magma chamber into fractures in the country rock and in the intrusion, cooling there and forming veins. Heat which is expelled from the intrusion can also cause water to be released from the country rock itself, which would very possibly begin to circulate through fractures in the rocks and could carry the minerals in solution, changing the area of depositing wherever it finds an open space. the circulating water can also move into porous country rocks, like sandstones or carbonates as an example, leaving minerals behind that have come out of solution as the fluids encoutered different chemical conditions or lost heat. as always, circulating hydrothermal fluids have a reaction with the rocks as they pass through, which causes alteration.