PEGMATITES

Location
The pegmatites are found in two geological units: the Grenville-age (1300 - 1000 Ma) metamorphic rocks of the Adirondack Mountains and the Taconic-age (450 Ma) metamorphic rocks of the Manhattan Prong (map). There are around 25 important pegmatite bodies that can be grouped as following:

  1. The Bedford pegmatite district consisting of the amazonite- and the peristerite-bearing pegmatites at Valhalla both in Westchester County, and the small xenotime- or beryl- and chrysoberyl-bearing pegmatite bodies in New York City. These pegmatites are emplaced in the metamorphic rocks of Taconic-age of the Manhattan Prong; all the remaining pegmatites listed below were emplaced in Grenville-age metamorphic rocks;
  2. The Cranberry Creek (Mayfield), Batchellerville, Greenfield, Day (Overlook), Corinth, and Wells belt of pegmatites in the southern Adirondack Highlands;
  3. The Crown Point, Rose Rock, Spar Bed Hill, Mineville, Chestertown, and Fort Ann pegmatites in the central-eastern Adirondack Highlands;
  4. The Scott Farm and Benson Mines pegmatites in the northwestern Adirondack Highlands; and
  5. The McLear and Rossie pegmatites in the Adirondack Lowlands.

History
Interest in the pegmatites of New York commenced with the rush to find and mine high-quality feldspar in the second half of the nineteenth century. The first attempt to mine it in New York State was around 1878 at Bedford. The mining lasted until 1949, and in 1962 and 1963 the dumps and most of the mine structures were leveled to build houses (Tan 1966).

In the Adirondacks, the Batchellerville pegmatite was discovered on the Adelbert Gordon Farm (Saratoga County) and opened in 1906 by Clapska Mining Company from Trenton, New Jersey. The pegmatite was worked until 1921 and probably closed by 1934 or before because no mining activity was reported after 1934. The main product was high-quality microcline for the ceramic industry (Tan 1966). A secondary product, very coarse muscovite, was typically sought after as a dielectric material.  However, iron oxide inclusions and microgranular staining made it unsuitable to electrical applications. The Cranberry Creek (Mayfield) pegmatite on the Richard Tyrell Farm was exploited at about the same time as Batchellerville pegmatite by the Clapska Mining Company, which later transferred it to the United States Feldspar Corporation (Tan 1966). The mining stopped here a few years before 1916. The Overlook (Day) body was worked until 1920.

The pegmatites from the East Central Adirondacks were first mined for enamel, crushed stone for concrete structures, chicken grit, and on a small scale for quartz used in glass manufacturing. Mining began before the beginning of the twentieth century and lasted probably until 1926 (Tan 1966).

In the northwestern Adirondacks, the McLear pegmatite was discovered in 1907. Here, feldspar was found in high-quality masses 6 in to 3 feet in length with no iron staining, but with rare grains of pyrite as inclusions. It was worked by the White Hill Mineral Company until 1937 and after that by the Green Hill Mining Company for feldspar that was shipped to Trenton, New Jersey, for making pottery. The mine was closed around 1938 (Tan 1966).

Mineralogy and classification
The pegmatites from New York have a simple to complex mineral composition. The pegmatites from Valhalla and New York City as well as those from the Adirondack Lowlands do not show compositional zoning.  Such zoning is a common feature of the Bedford pegmatites and those in the southern and eastern Adirondack Highlands, except for the Sugar Hill and Barton Hill pegmatites. Post-emplacement metamorphic deformation features are commonly observed in the pegmatites from the Lowlands and less commonly in the pegmatites from the Highlands. The pegmatites from southern New York lack even weak metamorphic features. By world-wide standards, the list of mineral species found in New York pegmatites is modest. At some locations secondary minerals formed at the expense of some of the primary ones; not all of the secondary minerals, especially the uranium-bearing minerals have been well identified and characterized.

The most widely used pegmatite classification today is based on the depth of emplacement, metamorphic grade, and minor element content (Cerny, 1991). Most of the pegmatites of New York fit into either Cerny's Rare-Element class (low-temperature and pressure), NYF-type (Niobium-Yttrium-Fluorine) or his abyssal (low- to high-pressure granulite facies) to muscovite (high-pressure kyanite-sillimanite Barrovian amphibolite facies) classes. These three categories partially overlap in mineral composition but differ by their relation to granitic bodies. Some minerals such as fluoro-edenite, fluorine-rich tremolite, and danburite found in the McLear pegmatite hinder classification of this pegmatite using this schema. Because the dominant mineral is microcline we consider that the best classification is probably the abyssal class of potassium feldspar-type; the Rossie, Sugar Hill, and Valhalla pegmatites could be other candidates for this class. The McLear and Rossie pegmatites were emplaced into the Lower Marble Formation of the Lowlands where the metamorphism reached the upper amphibolite facies. The presence of the Ca- and F-dominant/rich amphiboles and diopside could probably be explained by the contamination of the pegmatitic fluid with elements derived from the host siliceous marble.

Some pegmatites from the southern Adirondack Highlands (Batchellerville, Mayfield, and Greenfield) and New York City contain Al-rich silicates with or without Be and B such as sillimanite, beryl, chrysoberyl, dumortierite, and schorl as well as monazite and uraninite. They do not show any relationship with granite intrusions. The Batchellerville and Mayfield pegmatites are clusters of small bodies; the Greenfield pegmatite is a singular small body. They have the characteristics of the Cerny's muscovite class and may have formed form from the in situ partial melts rather than as a late-stage magmatic intrusion. According to their mineral composition Allanite, polycrase, columbite, titanite, zircon, fluorite, microcline1, and albite) the Scott's Farm, Crown Point, Roe Spar Bed, Day (Overlook), Lewis from the Adirondack Highlands, and Bedford and some of the small pegmatite bodies from the New York City could be part of the REE class NYF-type.


References

Cerny, P., 1991. Rare-element granite pegmatites. I. Anatomy and internal evolution of pegmatite deposits. Geoscience Canada, v. 18, p. 49-67.

London, D., 2008. PEGMATITES. The Canadian Mineralogist, Special Publication 10. 347 p.

Lupulescu, M., 2007. Minerals from New York State Pegmatites. Rocks & Minerals, V 82, No 6, p. 494-500.

Tan, L-P, 1966, Major pegmatite deposits of New York State. New York State Museum and Science Service Bulletin 408, 138 p.

 

 

 




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