Heat from the magma body will heat the surrounding groundwater, causing it to expand and then rise toward the surface. In some cases, this may initiate a convection system where groundwater circulates past the pluton. Such a system could operate for many thousands of years, resulting in the circulation of billions of litres of groundwater from the surrounding region past the pluton. Hot water circulating through the rocks can lead to significant changes in the mineralogy of the rock, including alteration of feldspars to clays, and deposition of quartz, calcite, and other minerals in fractures and other open spaces Figure 7.
As with the magmatic fluids, the nature of this circulating groundwater can also change adjacent to, or above, the pluton, resulting in deposition of other minerals, including ore minerals. Metamorphism in which much of the change is derived from fluids passing through the rock is known as metasomatism.
When hot water contributes to changes in rocks, including mineral alteration and formation of veins, it is known as hydrothermal alteration. A special type of metasomatism can take place where a hot pluton intrudes into carbonate rock such as limestone. When magmatic fluids rich in silica, calcium, magnesium, iron, and other elements flow through the carbonate rock, their chemistry can change dramatically, resulting in the deposition of minerals that would not normally exist in either the igneous rock or limestone.
These include garnet, epidote another silicate , magnetite, pyroxene, and a variety of copper and other minerals Figure 7. This type of metamorphism is known as skarn , and again, some important types of mineral deposits can form this way.
The heat from a body of magma in the upper crust can create a very dynamic situation with geologically interesting and economically important implications. In the simplest cases, water does not play a big role, and the main process is heat transfer from the pluton to the surrounding rock, creating a zone of contact metamorphism Figure In many cases, however, water is released from the magma body as crystallization takes place, and this water is dispersed along fractures in the surrounding rock Figure The water released from a magma chamber is typically rich in dissolved minerals.
As this water cools, it interacts with the surrounding rocks, changing both the chemistry of the water and the chemistry of the rocks. This can cause minerals to precipitate from the water.
Minerals can also precipitate if the water boils because of a drop in pressure. The precipitated minerals form veins within fractures in the surrounding rock. Quartz veins are commonly formed in this situation, and can include other minerals such as pyrite, hematite, calcite, and even silver and gold. Michard A Rare earth element systematics in hydrothermal fluids.
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