Friday, March 11, 2011

Unusual Columnar Jointing in Rocks Revealed at the Cresson Surface Mine

An extraordinary display of rock columns, formed from prehistoric magma (molten rock) that cooled underground, has been recently exposed by mining operations at the famous Cresson mine. The Cresson mine is located between Cripple Creek and Victor, Colorado. The Cripple Creek and Victor Gold Mining Company unearthed these magnificent rock columns while conducting routine mining operations at the surface mine.

About 32 million years ago a volcanic complex, with several eruptive vents formed along a deep break in the surface, was emplaced near Cripple Creek. Following the emplacement of the volcanic complex, mineral-rich fluids moved up from great depths and seeped into the fractures and fissures created by the violent volcanic upheaval, and cooled into hard, ore-bearing veins. This process formed a low-grade ore body of microscopic native gold attached to pyrite. Narrow, high-grade gold veins bearing quartz, pyrite, and fluorite were also formed. Most of the gold mined in the early days of the mining District came from the high-grade gold veins.

The historic Cresson mine began operations in 1906 with fair results. In 1914, the Cresson vug, or cavern, was discovered 1,200 feet below the surface of the mine. The room-sized vug was a rich strike—yielding over 60,000 troy ounces of gold in less than 4 weeks of frantic mining.

The potential of the Cresson deposit as a surface mine was recognized in 1990, and modern surface mining began in December 1994 to recover low-grade gold. The first gold ingot was poured in February 1995, and by the end of the year gold production was 76,500 ounces. The Cripple Creek and Victor Gold Mining Company continue mining operations at the Cresson surface mine. More than 250,000 troy ounces of gold were mined in 2008.

Recently, a remarkable display of columnar jointing was unearthed at the mine. The columnar jointing was formed as part of a body of magma that cooled underground into a rock known as phonolite.
Mining operations at the Cresson Surface mine exposed the geometric design of columnar joints, formed by a cooling mass of magma over 32 million years ago.  The mine is located north of Highway 67 between Victor and Cripple Creek, Colorado. 

The columnar joints found at the mine are parallel, prismatic columns that had formed in shallow magma at the mine. When these molten rocks cooled rapidly from the outside toward the center, they contracted. As a consequence, the shrinkage produced cracks or joints, generally in a hexagonal pattern that relived stress. Once the cracks or joints developed, they continued to grow, generally forming straight columns with parallel sides.

Columns typically form at right angles to the cooling surface where the molten rock makes contact. The size of columns depends on the rate of cooling of the rock—the faster the cooling, the smaller the columns.
As molten rocks cool below ground, they may shrink, forming joints.
Anywhere rocks that were once molten occur is a likely place for columnar jointing to develop: Devils Postpile in California, Sheepeaters Cliffs in Yellowstone National Park, and Devils Tower in Wyoming are good examples of these features.

Although such rock formations are often linked with legends of fearsome giants or the devil, there is nothing supernatural about them; they are simply geometric expressions of natural rock forming processes and are another example of the many geologic features in the Pikes Peak region.

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