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May-June 2012

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Montana Mineral Locality Index

KNOWN AS THE TREASURE STATE, Montana is rich in mineral and gemstone localities. The state motto, Ora y Plata meaning “gold and silver,” and the state seal with its representation of mining tools clearly proclaim Montana's mining origins. Montana's production of copper, coal, gold, lead, manganese, phosphate, silver, talc, vermiculite, and zinc is impressive. The official state fossil is Maiasaura peeblesorum, more commonly known as the duckbill dinosaur. Although a state mineral has not been designated, there are two official state gemstones: agate and sapphire. An extensive body of gem and mineral literature has been published describing the occurrence, collecting, mining, and cutting of the two state gemstones.

Geologic Provinces

The state can be divided into four major geologic provinces. The eastern third is dominated by sedimentary rocks and gentle terrain where crystals of barite, calcite, and gypsum are found along with petrified wood and agate. The middle third of the state is known as the Montana alkali province where the sediments have been penetrated by igneous rocks. These rocks contain both rare minerals and interesting ore deposits. The western third covers two provinces, the Rocky Mountains, and furthest to the northwest, the belt basin. The Rocky Mountains contain sedimentary, igneous, and metamorphic rocks of many types. This area of the state contains skarns, pegmatites, and metallic deposits and is a prolific source of mineral specimens. The belt basin is dominated by ancient metasediments called the Belt Supergroup. It is a sequence of argillite, siltite, quartzite, and carbonates nearly 50,000 feet thick; where these have been intruded by igneous rocks, metallic and industrial mineral deposits are found.

Michael J. Gobla holds degress in mining engineering, a bachelor's from the Colorado School of Mines and a master's from New Mexico Tech. He has more than thirty years' experience in mining, civil, and environmental engineering and began collecting Montana minerals in 1978.

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