Mineral Resources of the Appalachian/Piedmont

Figure 6.9: Principal mineral-producing localities of the Appalachian/Piedmont. Figure adapted from 1998 United States Geological Survey State Mineral Information.

Though most of the mineral mining in the Appalachian/Piedmont stopped before the early 1900’s, there are still several principal mining localities in the region producing zinc, aluminum, titanium, talc and mica (Figure 6.9). Other important mineral resources of the Appalachian/Piedmont (though not currently mined) include: the kaolin of the Precambrian Grenville rocks; the Ultramafic Belt chrome and asbestos, formed from metamorphosed serpentinite when the Taconic volcanic islands collided with North America; and copper and magnetite deposits of the Triassic Rift Basin.

The Grenville and Serpentine rocks of the Ultramafic Belt, and the Rift Basins of the Appalachian/Piedmont, host a plethora of non-metallic mineral resources in addition to the metallic minerals. 

Metallic Minerals in Grenville Rocks

Figure 6.10: Precambrian Grenville rocks of the Appalachian/Piedmont.

The Precambrian Grenville rocks of the Appalachian/Piedmont region, located along the spine of the Appalachians, peek through the sedimentary rock cover such as in the Adirondacks of the Inland Basin region (Figure 6.10). Associated with the Grenville rocks in Pennsylvania and Maryland are significant deposits of zinc ore, in its most common form, sphalerite (ZnS). At Franklin Furnace and Sterling Hill, New Jersey, zinc ore in the Grenville rocks is also found, though the ore minerals are unusual.

Metallic Minerals in Serpentine Rocks

Figure 6.11: Ultramafic Belt rocks in the Appalachian/Piedmont.

The Ultramafic Belt that extends the length of the Appalachian/Piedmont region from Vermont to Maryland contains a variety of minerals unique to the serpentinite rock found in the belt (Figure 6.11). The serpentinite rock itself is unusual, produced from the alteration of peridotites by metamorphism. The peridotite, derived from magma from the upper mantle of the Earth, was originally part of the oceanic crust. However, as the North American tectonic plate and the Taconic volcanic islands gradually drew closer together, the intervening oceanic crust was being pushed beneath the North American plate. Some of the oceanic crust was scraped off and welded onto the side of North America as the rest of the oceanic crust was shoved down into the mantle. The peridotite of the oceanic crust was metamorphosed to form serpentinite, a rock rich in minerals not often found as part of the continental crust. A metallic mineral of note in the serpentinite rocks is chromite. The only ore of chromium, chromite was at one time mined in the Ultramafic Belt serpentinite rocks of Pennsylvania and Maryland. A dense, heavy mineral, chromite is one of the first minerals to crystallize and settle to the bottom of a cooling magma. It was thus concentrated in the serpentinite rocks in quantities sufficient to be profitably mined.

Metallic Minerals in Rift Basin Rocks

Figure 6.12: Triassic Rift Basins in the Appalachian/Piedmont.

The Newark and Gettysburg Triassic rift basins of the Appalachian/Piedmont region stretch through southeastern New York, New Jersey, Pennsylvania and Maryland. Formed during the rifting of Pangea away from North America, the rift basins contain alternating layers of igneous and sedimentary rocks (Figure 6.12). The resistant, ridge-forming igneous rocks, produced from lava flows (basalt) or igneous intrusions (diabase), contain mineral resources of economic importance to the region.

In particular, magnetite (Fe₃ O₄) is an important mineral resource in the Pennsylvania and New Jersey diabase rocks, concentrated and subsequently precipitated by hot flowing water through the rocks. Magnetite is one of the common ores of iron. Copper deposits are also associated with the basalt lavas of the rift basins.

Metallic Minerals in Other Rocks

Other important metallic minerals in the Appalachian/Piedmont region include nickel, molybdenum, titanium, manganese, cobalt, and graphite. In northern Delaware, titanium is an important mineral resource associated with the igneous rocks of the area, mined commercially for use as a paint pigment. In the Piedmont, gold is found in small quantities associated with quartz veins and fault zones in the metamorphic rocks of the region. Sillimanite (Al₂SiO₅), Delaware’s state mineral, is found in the Appalachian/Piedmont region of northern Delaware as large crystals produced from aluminum-rich rocks that were deeply buried and subjected to intense metamorphism. Though the mineral is not limited to Delaware, the unusually large crystals of sillimanite found there are rare elsewhere.

Non-Metallic Minerals in Grenville Rocks

There is an abundance of non-metallic mineral resources in the billion year-old Grenville rocks, including mica, feldspar and quartz. Mica, a common mineral in igneous, metamorphic and sedimentary rocks, is mined in southern Pennsylvania in Adams County from the Precambrian Grenville rocks that form South Mountain. Kaolinite (Al₂Si₂O₅(OH)₄), a white clay formed from the weathering of feldspar, is mined in Vermont.

Non-Metallic Minerals in Serpentine Rocks

The Ultramafic Belt of serpentinite contains at least two important associated non-metallic minerals, which commonly form through the metamorphism of the magnesium-rich rocks: asbestos (Mg₃Si₂O₅(OH)₄) and talc (Mg₃Si₄O₁₀(OH)₂). At one time, Vermont produced the most asbestos in the United States, though it is no longer mined there. Talc continues to be mined in Vermont today. An extremely soft mineral, talc can be scratched easily with your fingernail and has a soapy, greasy feel typical of very soft minerals.

Non-Metallic Minerals in Rift Basin Rocks

The Triassic Rift Basin of the Appalachian/Piedmont also has its share of non-metallic minerals. Basalt, formed as lava broke out of the crust and flowed across the surface of the basin, cooled quickly, trapping gas bubbles within the rock that left small cavities. Later, as water flowed through the rock, minerals were precipitated in the cavities, forming crystals such as the green mineral prehnite (Ca₂Al(AlSi₃O₁₀)(OH)₂). Paterson and Bergen Hill, New Jersey are known for this mineral.

Non-Metallic Minerals in Gemstones

In addition to the non-metallic minerals discussed above, the Appalachian/Piedmont region produces several types of gemstones. The very common mineral feldspar has several relatively rare varieties found in Pennsylvania that are sold as the gemstones sunstone and moonstone. Amethyst, smoky quartz, agate, garnet and beryl are also found in the region. Beryl (Be₃Al₂(Si₆O₁₈)) is common in granites and pegmatites and comes in a variety of colors.