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Iron is the most abundant chromophore (an element that imparts color to a mineral by its presence in the struc-ture) in the earth. When iron is in its divalent oxidation

state (Fe2+), as is found in such minerals as olivine, pyrox-enes, amphiboles, chlorites, and epidote, it typically imparts a green color. Exceptions to this do exist and include, for example, almandine, which is colored red by the presence of divalent iron.

Acting as does pigmentation in paint, Fe2+-bearing min-erals, even as minor constituents in a rock, can impart a distinct green color, especially when fine grained and highly dispersed. Although many minerals and rocks are green stones (e.g., malachite, emerald, amphibolite, and dunite), the term greenstone is not used as a noun to refer to them. There are, however, several specific, but different, cases where the term greenstone is used.

In vesicular basalts from the Lake Superior region (rocks of the Keweenawan System), a variety of pumpellyite called chlorastrolite forms amygdules (Rakovan 2005) in compact, finely radiated or stellate masses (fig. 1). These amygdules are commonly referred to as Michigan greenstone (aka: greenstone and Isle Royale greenstone) and are also Michi-gan’s official state gemstone. The individual stellate masses are often chatoyant: thus the name chlorastrolite, meaning “green star stone” (Heinrich and Robinson 2004).

In New Zealand, greenstone is used to describe nephrite-jade, which is found on the Southern Island in the Tara-makau-Arahura and Wakatipu regions (fig. 2). It has been used extensively by the Maori for weapons and ornaments since before Western colonization and today is a popular gemstone. The Maori name for nephrite is pounamu. The term greenstone is also applied by some to the bowenite variety of serpentine (Maori: tangiwai), which is found at Anita Bay in Milford Sound, New Zealand (McLintock 1966; Beck 1991).

In their article on the Mockingbird gold mine, Mariposa County, California, Cook and Gressman (2008) mention greenstone; however, the greenstone to which they refer is, a third, and certainly the most widely used, example of a greenstone. It is a very fine-grained, altered, or metamor-

phosed mafic igneous rock (e.g., basalt, gabbro, or diabase) that lacks a foliated texture and is green in color (commonly a drab gray-green) due to the presence of some combination of chlorite, actinolite, epidote, zoisite, prehnite, or pumpel-lyite (fig. 3). This mineral assemblage, along with albite, sphene, and in some cases a carbonate mineral, can partially to completely replace the original magmatic minerals during metamorphism.

Although they are not normally qualified by the use of

JOHN RAKOVANDepartment of GeologyMiami UniversityOxford, Ohio 45056rakovajf@muohio.edu

Dr. John Rakovan, an executive editor of Rocks & Minerals, is a professor of mineralogy and geochemistry at Miami University in Oxford, Ohio.

Greenstone

Word to the Wise

Figure 1. Michigan greenstone (chlorastrolite) from Isle Royale. The amygdule width is 2.5 cm. Seaman Mineral Museum speci-men, John Jaszczak photo.

Figure 2. New Zealand greenstone (nephrite-jade) pendant, tradi-tional fishhook design (Maori: hei-matau). Fashioned in Christchurch, New Zealand.

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554 ROCKS & MINERALS

tural features and their relationships to surrounding rocks, have been interpreted as being the result of changes in the dynamics of plate tectonics throughout Earth’s history (Best 1982; De Wit and Ashwal 1997).

Gold is found in greenstone belts throughout the world and is thought to be mobilized by hydrothermal solutions during regional metamorphism. Emplacement is usually in quartz veins or disseminated in adjacent altered rock. The deposits of the Golden Mile in Kalgoorlie, Western Australia, are a classic example of greenstone belt–related gold; they have produced more than 46 million ounces of gold (Bateman and Bierlein 2007). Another example is the Barberton greenstone belt that hosts the oldest recognized orogenic (associated with mountain-building) gold ores on Earth. In the Barberton, most of the gold deposits lie within greenschist facies metamorphic rocks, and it has been sug-gested that hydrothermal emplacement of the gold is related to the intrusion of granites and syenites that are associated with the belt (Foster and Piper 1993).

Given the preponderance of green in the natural world, including the mineral kingdom, it comes as no surprise that the term greenstone has found common usage in describ-ing different green stones. As pointed out by Dr. Bill Cor-dua, University of Wisconsin–River Falls, an amusing cir-cumstance of nomenclature and geology is that Michigan greenstone (masses of chlorastrolite) is sometimes found in basalts that have been metasomatized into metamorphic greenstones. In other words, we can find greenstones in greenstone.

AKNOWLEDGMENTSI thank Kendall Hauer and Peter Modreski for their reviews of

this article.

REFERENCESBateman, R., and F. P. Bierlein. 2007. On Kalgoorlie (Australia),

Timmins-Porcupine (Canada), and factors in intense gold min-eralization. Ore Geology Reviews 32:187–206.

Beck, R. 1991. Jade in the South Pacific. In Jade, ed. R. Keverne, 222–57. New York: Van Nostrand Reinhold.

Best, M. G. 1982. Igneous and metamorphic petrology. New York:W. H. Freeman and Company.

Cook, R. B., and T. M. Gressman. 2008. The Mockingbird gold mine, Mariposa County, California. Rocks & Minerals 83:392–401.

De Wit, M., and L. D. Ashwal, eds. 1997. Greenstone belts. Oxford monograph on geology and geophysics 35. Oxford: Clarendon Press.

Foster, R. P., and D. P. Piper. 1993. Archaean lode gold deposits in Africa: Crustal setting, metallogenesis and cratonization. Ore Geology Reviews 8:303–47.

Heinrich, E. W., and G. W. Robinson. 2004. Mineralogy of Michigan. 2nd ed. Houghton: A. E. Seaman Mineral Museum, Michigan Technological University.

Herrington, R. J., D. M. Evans, and D. L. Buchanman. 1997. Metal-logenic aspects. In Greenstone belts, ed. M. De Wit and L. D. Ashwal, 176–219. Oxford monograph on geology and geophysics 35. Oxford: Clarendon Press.

McLintock, A. H., ed. 1966. An Encyclopaedia of New Zealand. Te Ara—The Encyclopedia of New Zealand, updated 18-Sep-2007. URL: http://www.TeAra.govt.nz/1966/G/Greenstone/en.

Rakovan, J. 2005. Word to the Wise: Amygdule. Rocks & Minerals 80:287. q

metamorphic, to avoid confusion I will use this adjective to describe the third usage of greenstone. Metamorphic greenstones occur worldwide in rocks of essentially all ages. Best (1982) speculates that most metamorphic greenstones lie buried beneath the ocean floors, where they were formed by metamorphism of the oceanic crust, made up of basalts and deeper crystallized gabbros. The grade (pressure-tem-perature regime) of alteration in metamorphic greenstone is low, typically falling within what is known as the greenschist facies (Best 1982). Because of the low grade of metamor-phism, relict magmatic fabrics, such as pillow structures, are often preserved.

Metamorphic greenstones also comprise, to varying degrees, large rock sequences of Precambrian age (Archaean through Proterozoic) that are called greenstone belts. Such belts also include metamorphosed sedimentary rocks and are most often associated with granites and gneisses. It is generally agreed that these assemblages formed at ancient plate boundaries including oceanic spreading zones, such as today’s Mid-Atlantic Ridge, and island arcs, such as the Mariana Islands in the Western Pacific (De Wit and Ash-wal 1997). Examples include the Barberton greenstone belt (South Africa), the Abitibi and Temagami greenstone belts (Ontario, Canada), the Isua greenstone belt (southwestern Greenland), the South Pass greenstone belt (Wind River Mountains, Wyoming), and the Marquette greenstone belt (Upper Peninsula, Michigan).

Greenstone belts are significant for several reasons, not the least of which is that they are a window into the crustal evolution and the plate-tectonic history of early Earth. They are also host to significant metal deposits including copper, iron, and gold (Herrington, Evans, and Buchanan 1997). Differences in the nature of greenstone belts that formed during the Archaean (3.8–2.5 billion years ago), Proterozoic (2.5 billion–543 million years ago), and Phanerozoic (543 million years ago to present), especially in terms of struc-

Figure 3. Greenstone erratic, 3 feet across, from Oxford, Ohio. Light-colored rounded areas are felsic xenoliths.

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