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Mineral Collecting at Contact Zones

What to Look For in Country Rock and Wall Rock


Once you have become familiar with the major rock types and their typical minerals, it's time to take your game up a level. As in other areas of life, unusual things take place at the edges. In geology, that's where different rock types meet.

Plutonic Invasions: Country Rock

A prime playground for mineral chemistry is the intersection where deep-seated magma invades the older rock around it, referred to as country rock. Magma introduces two things that can create new minerals: heat and fluids.

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.

Eruptive Disruptions: Wall Rocks

A more dynamic setting for finding special minerals is areas of present and past volcanism. Here, magma does not pool but flows instead through narrow underground channels, called dikes or sills. Dikes are sheetlike magma intrusions that cut across the grain of the country rock; sills lie along the grain, between the rock layers. In either case, they are worth examining because they can pluck away pieces of their wall rocks and carry them away as xenoliths.

Xenoliths may simply float intact in the magma, or they may melt into wisps. The most interesting ones to mineral collectors are in between: former wall rocks that undergo metamorphism in the magmatic conditions. A famous example is found at Italy's Vesuvio volcano, where blocks of limestone wall rock have been torn off, held in the shallow magma chamber, then thrown from the volcano's throat. The limestone minerals are changed by the heat to calcium-rich silicates like wollastonite, nepheline and garnet.

The xenoliths from deep in the mantle that are found in diamond pipes (kimberlites) are another. Most of us will never have the chance to collect from a kimberlite, because diamond prospectors invariably stake claims on them. But a closely related body of lamproite in Arkansas is open to the public at Crater of Diamonds State Park, where you can keep whatever you find.

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