Aureole is a ring around an igneous intrusion. Deep magma bodies cool very slowly and turn into coarse-grained, plutonic rocks like granite and gabbro. During this time they bake a zone, or aureole, of contact metamorphism in the country rock. Prolonged heat doesn't always melt the country rock, but it does mobilize the elements and allow new minerals to form and grow that are stable in the high temperatures. It also may help the existing minerals recrystallize into larger grains.

Usually the rocks in the aureole are fine-grained types classified as hornfels. Depending on the amount of heat available, the presence of groundwater and other factors, contact-metamorphic aureoles may be hundreds of meters thick or no wider than the length of a hammer handle. These rocks may grade through different degrees of mineral alteration, or contact-metamorphic facies: pyroxenes in the hotter parts,amphiboles farther away, and epidote along the outer rim. Late-breaking veins may further change these mineral assemblages.

Magma is not just a melt; it leaks water-rich fluids into its surroundings, and at high heat and pressure these can deeply alter country rock in the process called metasomatism. Large bodies of rock where igneous fluids alter sedimentary rocks are called skarns. Dramatic examples occur where granite invades limestone, but mudstones and shale (pelitic rocks) can be turned into unusual mineral mixtures too.

Finally, late-stage fluids can break out just before the plutonic intrusions finish turning into rock. These are especially well disposed to form large crystals and host rare minerals; the resulting bodies of very coarse grained minerals are called pegmatites. Miners as well as collectors seek these out.

BY Michael Yurosko, Pickax Mining & Exploration