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Mount Pleasant

The Devonian Period has had an impact on New Brunswick’s mineral industry. During this period of subduction and mountain building, less dense components of melting ocean-floor crust rose upward and solidified near the surface of the Earth. Magma and hot fluids rising in the crust also brought rare minerals including tungsten, molybdenum, wolframite, indium and antimony toward the surface with the plutons. Sometimes the magma reached the surface to produce volcanoes associated with the subduction of the ocean crust.

Colour graphic illustrating subduction and mountain building process.

The geology of Mount Pleasant Caldera Complex includes a diverse array of intrusive and extrusive rocks resulting from a long history of volcanic activity. Within the caldera rock types include ash flow tuffs, thick sedimentary layers full of angular fragments, and igneous rocks that intrude the volcanic pile. Surrounding the caldera are more ash flow tuffs, lavas, and river-lain sediments.

South of Fredericton at Mount Pleasant the discovery of an orebody has generated considerable mining interest since tin was found in 1937. The setting is in the Mount Pleasant Caldera Complex. In the Late Devonian upwellings of magma reached the surface and exploded to form a broad volcanic cone, while below granitic rocks slowly cooled. The volcanic cone eventually collapsed creating a caldera, a bowl-shaped depression at the surface. The chemistry of the magma caused the granites to be enriched in tungsten, molybdenum, tin and other metals. The ore body, called a polymetallic deposit, is said to contain more than one hundred kinds of minerals. It is North America’s largest tin deposit and has the world’s largest reserves of indium, used in electronics. The complex ore composition has made development of a mine very difficult, but the rewards for developing a successful mine could be high. Adex Mining Inc. is now investigating the Mount Pleasant deposit to develop a mine.

The geology of Mount Pleasant Caldera Complex includes a diverse array of intrusive and extrusive rocks resulting from a long history of volcanic activity. Veins with minor molybdenite mineralization cut the granite that once fed the volcano. Within the caldera rock types include ash flow tuffs, thick sedimentary layers full of angular fragments, and igneous rocks that intrude the volcanic pile. Surrounding the caldera are more ash flow tuffs, lavas, and river-lain sediments.