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Chapter 2. Geologic Settings of Maine Minerals
Most of the good mineral localities in Maine occur in one of several common geologic environments, such as granite pegmatites or metamorphic rocks. Each of these settings is briefly described here, so that you will have a better understanding of how the mineral deposits originated. The bedrock geology of the state is shown on the Simplified Bedrock Geologic Map of Maine (pdf file), a generalized version of the Bedrock Geologic Map of Maine (Osberg and others, 1985).
Granite pegmatites are a variety of granite in which the individual mineral grains are so large that they are measured in inches or even in feet. One of the largest of the gigantic beryl crystals found at the Bumpus Quarry (Albany, Maine) in the late 1920's was 18 feet long, 4 feet in diameter, and estimated to weigh about 18 tons (Gedney and Berman, 1929). Figure 1 shows some of these crystals embedded in the quarry wall.
The pegmatites in southwestern Maine formed by the gradual cooling and solidification of fluids derived from nearby granites and other similar igneous rocks, or as a result of partial melting of the host rock during metamorphism. The fluids that formed the pegmatites tended to be enriched in water and rare elements. They often were injected into fractures in the surrounding rock and crystallized to form lenses or wall-shaped masses called "dikes." In some cases the resulting minerals are distributed rather evenly throughout most of the pegmatite. In contrast to these "simple pegmatites" are the "zoned pegmatites," in which there are two or more zones of different mineralogical composition (Cameron and others, 1949). The zones often formed in a roughly concentric pattern around the core of the pegmatite as crystallization proceeded inward from the margins.
The layered sequence of mineral assemblages that developed in zoned pegmatites is the same in many places, so the astute prospector can use this knowledge in searching for mineral specimens. There are also "replacement bodies" in some pegmatites, where chemical reactions formed secondary minerals by the alteration and replacement of those which had crystallized earlier. A number of the rare and desirable pegmatite minerals occur in this environment. Cameron and others (1954) have described the zonation and other characteristics of New England pegmatites in great detail. Landes (1925) discussed the several generations of minerals in complex Maine pegmatites, especially at the Bennett Quarry in Buckfield. The principal constituents of Maine pegmatites are the same minerals that form ordinary granite: quartz, feldspar, and mica. Concentrations of high-quality mica and feldspar have been discovered in many pegmatites, and these and other minerals have been exploited by means of open-pit quarries (Figure 2) and, less commonly, underground workings (Figure 3). Large, shiny fragments of pegmatite minerals often litter the roads leading to the quarries, and help point the way to collecting sites.
Quartz occurs in a variety of colors, but most of it is milky white to smoky gray or black. Rose quartz is found in some places, and Maine is one of the very few areas in the world where rose quartz crystals have been collected. Several species and varieties of feldspar and mica are present in the pegmatites. Most of the mica is either colorless to brownish muscovite (which was mined at numerous localities) or black biotite. Large flat sheets of biotite are especially common in pegmatites of the Topsham area. Lepidolite, a third mica species, often has an attractive lilac color. Mixtures of lepidolite with a platy white variety of albite feldspar (cleavelandite) may contain good specimens of colored tourmaline and other minerals.
Well-formed crystals of some pegmatite minerals may be tightly encased in the rock matrix. These include microcline feldspar, beryl, schorl (black tourmaline), almandine (garnet), and elbaite (pink, green, or blue tourmaline). The best crystals of these minerals tend to be surrounded by quartz which solidified around them. With care, the brittle quartz can be partly chipped away to expose a nice "matrix specimen." Such specimens are more interesting and valuable than crystals that have been totally detached from the host rock.
Under favorable conditions, pegmatite minerals grew into open cavities in the rock ("pockets" or "vugs") where they tended to achieve greater clarity and perfection than other crystals of the same species that are embedded in the matrix (Figure 4). The better-known minerals that may be found in pegmatite crystal pockets include quartz (colorless, smoky, amethyst, or rose), colored tourmaline, beryl (golden and aquamarine), apatite, and topaz. In the famed tourmaline mines at Newry and Paris, Maine, cavities large enough for a person to crawl into have been opened (Figure 5). Geological disturbances have usually caused many of the crystals to break loose from the walls, and they may now lie in a jumbled pile surrounded by clay on the cavity floor.
Minerals Found in Metamorphic Rocks
A lot of the bedrock in Maine is metamorphic rock, which formed by the recrystallization of older rocks (largely sedimentary and volcanic) that were buried and subjected to high temperatures and pressures deep in the Earth's crust. The degree of metamorphism is most intense in the southern part of the state, where there are large plutons of granite and other igneous rocks (Osberg and others, 1985). In northern Maine the bedrock is only slightly metamorphosed, and the original sedimentary layering is preserved.
Crystals of several mineral species may be found in metamorphic rocks, depending on the original compositon of the parent rock and the physical and chemical conditions that prevailed during metamorphism. Marine muds that had previously hardened to form a sedimentary rock called shale were converted to mica-rich rocks such as slate and schist. Mica schist is coarser-grained than slate, and developed where metamorphism was great enough to cause considerable recrystallization. Schists locally contain crystals of dark-red almandine (garnet), white to blue kyanite, gray or pink andalusite, or brown staurolite. The Cook Road staurolite locality in Windham (now closed) is an example of this kind of deposit. Schists tend to split more easily along directions in which the parallel mica flakes are aligned (planes of foliation). This property helps in breaking apart slabs of rock to expose the embedded crystals. The better crystals of minerals such as staurolite tend to be concentrated in layers in the schist.
Marble quarries are abundant in the Rockland-Thomaston, Rockport, and Union areas in Knox County. They once supported a booming industry based on the production of agricultural lime (Grindle, 1971), but most are now closed. One can locate the quarries by inquiring locally, and they are shown on maps by Morrill and Hinckley (1959) and Allen (1953). There is little published information on the mineral collecting potential of these marble quarries, but they have provided some good specimens of calcite crystals. Tremolite and other minerals also have been reported (Morrill and Hinckley, 1959).
In places where the original rock was an impure limestone, metamorphism produced a different assemblage of minerals. Among the more common minerals in this setting are orange grossular (a member of the garnet group), green diopside, and olive-brown vesuvianite. They usually occur as massive, interlocking grains that form a tough "calc-silicate" rock (also known as "lime-silicate" or "skarn"). Good crystals of these minerals formed in some cases, particularly where they are partly enclosed by calcite or quartz. Calcite has often been removed by weathering, or can be dissolved in acid to expose the crystals. Well-known localities for calc-silicate minerals include the Pitts-Tenney Quarry in Minot and the Sanford vesuvianite locality (Shaub, 1957; Leavitt, 1987; Leavitt and Leavitt, 1993). Like the occurrences in mica schist, most such localities are relatively small outcroppings or prospect pits. They are smaller than commercial pegmatite operations, having been worked only by mineral collectors. If you are looking for fine specimens, you should go prepared with heavy hammers, chisels, and other equipment needed for loosening and breaking large slabs of rock from the ledges.
Metal Sulfide Minerals
The sulfide minerals consist of various combinations of metals with the element sulfur. Sulfides of iron (pyrite, pyrrhotite), copper (chalcopyrite), zinc (sphalerite), and lead (galena) occur in Maine. These minerals are shiny and brittle, and they have a metallic luster on fresh surfaces. Pyrite ("fool's gold") is frequently crystallized, while the other sulfides are usually irregular masses.
The sulfides in Maine are found in several environments. They occur in metamorphosed sedimentary and volcanic rocks, veins along fault zones, and as sparsely disseminated grains in igneous rocks. Deposits of the above minerals have been mined in many places, notably in the belt of volcanic rocks in eastern coastal Maine. These mines are typically very small. Some of them were worked for the gold and silver that occur as minor impurities in the base-metal sulfides. There was a short-lived boom in the 1880's (Figure 6), which even resulted in publication of a weekly mining newspaper in Bangor (Figure 7), but low-grade ore and changing economic conditions soon forced the majority of the mines to close. A few, such as the Harborside Mine in Brooksville, were occasionally worked until recent times. Intensive exploration activity occurred in the 1960's (Figure 8), and has persisted intermittently through the present day. Large copper and zinc sulfide deposits have been discovered in northern Maine, but have yet to be developed into mines.
The piles of waste rock around old mines are usually the best places to collect sulfide ores. Hussey and Austin (1958) and Morrill and Hinckley (1959) list the metal mines in Maine, and more detailed information can be obtained from publications such as those by Young (1962, 1963). Some detective work and local inquiries will help in finding localities that are small and overgrown. Watch out for mine shafts as you explore these areas. Rusty pieces of rock on the mine dumps are a guide to sulfide mineralization, and you generally can obtain fresh specimens by cracking open the dump material.
Other Mineral Occurrences in Bedrock
In addition to the types of mineral deposits described above, there are other, less common occurrences in Maine. Exploration of these localities offers the potential for discovering an assortment of interesting minerals. Several examples are briefly discussed here, but there are almost certainly others waiting to be found.
A number of minerals besides the metal sulfides may be disseminated through igneous rocks. An outstanding example is the Litchfield area, where there is a type of syenite rock known as "litchfieldite." It is curious that this relatively small body of rock is almost totally concealed beneath glacial sediments; its breadth has been determined chiefly from glacially transported boulders that are scattered about the countryside. Some of these boulders contain attractive patches of bright blue sodalite and yellow cancrinite. Zircon crystals up to 1 inch long are also embedded in the rock. Granitic igneous rocks sometimes contain open fissures which are lined with crystals of quartz, feldspar, and other minerals. In spite of all the granite in Maine, these crystal pockets do not seem to be very large or abundant. They are much more prevalent in the younger Conway granite of neighboring New Hampshire (Samuelson and others, 1990). However, a few cavities containing crystals of smoky quartz have been found in the granite quarries of Hallowell and Stonington, and in road cuts.
Large pockets of quartz crystals may be encountered in fault zones and other structurally controlled cavities. One such pocket was discovered in a road cut along U.S. Route 2 in Mexico, Maine, and produced many pounds of doubly-terminated quartz crystals. Another road cut - on U.S. Route 1 in Calais - yielded a rare find of sharp prehnite crystals on crusts of quartz crystals. In 1987, the excavation of a pit for road fill in Sweden, Maine, exposed fractured bedrock with cavities containing spectacular deep-purple amethyst crystals on a matrix of white quartz crystals (Thompson, 1990). Basalt is a fine-grained, dark green to black igneous rock that outcrops in the coastal and northern Maine volcanic belts. These areas have not been thoroughly checked for mineral specimens, but some of the gas pockets that formed in the cooling volcanic magmas contain crystals of various minerals, as well as agate nodules. The outcrops at the Perry agate locality and other sea cliffs in the eastern part of the Maine coast belong to this category.
The mineralogy of northern Maine has received little attention because of its remote location and the appeal of the southern Maine pegmatites. It is likely that the adventurous collector could find some good specimens of minerals and fossils in Aroostook County and elsewhere in northern Maine. Most of the bedrock in this region consists of weakly metamorphosed sedimentary and volcanic rocks. There are lime quarries in the Aroostook County agricultural belt, where calcite crystals and other minerals may occur. Large deposits of unusual iron and manganese-rich bedrock have been discovered in several places (Miller, 1947). The manganese prospects on Maple and Hovey Mountains (west of Bridgewater) contain an assortment of minerals, including braunite, hematite, magnetite, rhodochrosite, and rhodonite (Pavlides, 1962).
Alluvial and Beach Deposits
Alluvial mineral deposits, also known as placers, are those which occur in sand and gravel laid down by running water. "Alluvium" includes both modern stream sediments and those deposited during relatively recent geologic time (as opposed to the older, indurated sedimentary rocks). The alluvial deposits of most interest in Maine are the gold placers. Gold has been panned for many years from the Swift River in Byron, and from other lesser-known streams. It seems probable that gold could also be found in the older sand and gravel deposits left by glacial meltwater streams, but there are no published accounts of such discoveries in Maine.
Gravel beaches occur in many places along the Maine coast. They contain pebbles and cobbles that have been eroded from nearby bedrock or glacial sediments and rounded by tumbling in the surf zone. Beachcombing in these areas is a popular pastime. Good crystals rarely survive in the storm-ravaged environment of gravel beaches, but one may find brightly colored stones that are suitable for lapidary work and ornamental uses. Jasper Beach in Machiasport is a favorite collecting site, though the so-called "jasper" is actually a mottled reddish-brown volcanic rock. A variety of unusual beach cobbles can be collected at this and other localities in the granitic/volcanic terrain of eastern coastal Maine.
Allen, H. W., 1953, Progress report of limestone survey - Knox County, in Report of the State Geologist, 1951-52: Augusta, Maine, Maine Development Commission, p. 11-29.
Cameron, E. N., Jahns, R.H., McNair, A. H., and Page L. R., 1949, Internal structure of granite pegmatites: Economic Geology, Monograph 2, 115 p.
Cameron, E. N., Larabee, D. M., McNair, A. H., Page, J. J., Shainin, V. E., and Stewart, G. W., 1954, Pegmatite investigations, 1942-45, New England: U.S. Geological Survey, Professional Paper 255, 352 p.
Gedney, E. K., and Berman, H., 1929, Huge beryl crystals at Albany, Maine: Rocks and Minerals, v. 4, no. 3, p. 78-79.
Grindle, R. L., 1971, Quarry and kiln - the story of Maine's lime industry: Rockland, Maine, Courier-Gazette, 331 p.
Hussey, A. M., II, and Austin, M. B., 1958, Maine metal mines and prospects: Augusta, Maine, Maine Geological Survey, Minerals Resources Index No. 3, 53 p.
Landes, K. K., 1925, The paragenesis of the granite pegmatites of central Maine: American Mineralogist, v. 10, no. 11, p. 355-411.
Leavitt, D. L., 1987, Skarns of southwestern Maine: Rocks and Minerals, v. 62, no. 6, p. 429-432.
Leavitt, D. L., and Leavitt, N. J., 1993, Mineralogy of the Sanford vesuvianite deposit: Mineralogical Record, v. 24, p. 359-364.
Miller, R. L., 1947, Manganese deposits of Aroostook County, Maine: Augusta, Maine, Maine Development Commission - Maine Geological Survey, Bulletin 4, 77 p.
Morrill, P., and Hinckley, W. P., 1959, Maine mines and minerals, Vol. 2, eastern Maine: privately published, 80 p.
Osberg, P. H., Hussey, A. M., II, and Boone, G. M., 1985, Bedrock geologic map of Maine: Augusta, Maine, Maine Geological Survey, 1:500,000-scale map.
Pavlides, L., 1962, Geology and manganese deposits of the Maple and Hovey Mountains area, Aroostook County, Maine: U.S. Geological Survey, Professional Paper 362, 116 p.
Samuelson, P. B., Hollmann, K. H., and Holt, C. L., 1990, Minerals of the Conway and Mount Osceola granites of New Hampshire: Rocks and Minerals, v. 65, no. 4, p. 286-296.
Shaub, B. M., 1957, Garnet locality of Minot, Maine: Rocks and Minerals, v. 32, nos. 5-6, p. 227-234.
Thompson, W. B., 1990, Amethyst deposit found at Sweden, Maine: Mineral News, v. 6, no. 4, p. 1-2.
Young, R. S., 1962, Prospect evaluations, Hancock County, Maine: Augusta, Maine, Maine Geological Survey, Special Economic Studies Series No. 2, 113 p.
Young, R. S., 1963, Prospect evaluations, Washington County, Maine: Augusta, Maine, Maine Geological Survey, Special Economic Studies Series No. 3, 86 p.
Last updated on October 6, 2005
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