Mineral Resources of the Exotic Terrane

Overview

Metallic Resources

With the collision of the Taconic volcanic islands in the Ordovician, Baltica and Avalonia in the Devonian, and the final collision with Africa during the Permian, the various slices of the Exotic Terrane region have undergone significant periods of compression, deformation, metamorphism and intrusion by magmas. These dynamic geologic conditions gave rise to the formation of many metallic minerals (often associated with igneous and metamorphic rocks). Granite pegmatites, common in this area, often include uranium, gold, antimony, graphite, and iron. Gold, lead, silver and copper are associated with the metamorphic rocks in the region and commonly found in association with one another.

Exceptionally fine quality crystals of pyrite, ‘fools gold,’ are found in Chester, Vermont. A very common and widespread mineral, pyrite forms in igneous, metamorphic and sedimentary environments as well as through the chemical alteration of other minerals. Other metallic minerals and ores found in the Exotic Terrane region include molybdenum, cobalt, nickel, tin, and tungsten. The largest manganese deposit on the North American continent is found in Maine in Silurian rocks of Aroostook County. The mildly metamorphosed Silurian rocks were once sediments at the bottom of the Iapetus Ocean. Concentrations of manganese commonly form on ocean bottoms today.

Non-Metallic Resources

The primary non-metallic minerals of the region are metamorphic and pegmatite minerals, which are so common in the Exotic Terrane area because of the foundation of igneous and metamorphic rock.

Metamorphic rocks

The process of metamorphism ranges from low grade (with only mild increases in pressure and temperature) to high grade (with severe increases in pressure and temperature). Mildly deformed rocks may be subjected to very low grades of metamorphism and outwardly exhibit little change in appearance. Severely deformed rocks, on the other hand, have usually been subjected to very high grades of metamorphism and appear distinctly different. The higher the degree of metamorphism, the greater the change is to the original rock. The changes include, to varying degrees, the alignment of minerals within the rock, recrystallization of minerals, and, in many cases, the crystallization of new minerals.

Geologists have determined that specific minerals will form at specific temperature and pressure conditions when a given type of rock is metamorphosed. Low-grade metamorphism of clay-rich rocks such as shale, produce the mineral chlorite. Higher-grade increased metamorphism produces the minerals kyanite and sillimanite. The minerals associated with certain grades of metamorphism are known as index minerals, indicative of the combination of the temperature and pressure conditions a rock has undergone.