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Home , Amphibolite, Chlorastrolite, Diabase, Gabbro, Greenschist, Greenstone belt

Word to the Wise JOHN RAKOVAN Department of Miami University Oxford, Ohio 45056 [email protected] Greenstone ron is the most abundant chromophore (an element that imparts color to a by its presence in the struc- ture) in the . When is in its divalent oxidation I 2+ state (Fe ), as is found in such as , pyrox- enes, , chlorites, and , 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 , 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, , and ), 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. Figure 1. greenstone (chlorastrolite) from . The amygdule width is 2.5 cm. Seaman Mineral Museum speci- In vesicular from the region (rocks men, John Jaszczak photo. of the Keweenawan System), a variety of called chlorastrolite forms amygdules (Rakovan 2005) in compact, phosed (e.g., , , or ) finely radiated or stellate masses (fig. 1). These amygdules that lacks a foliated texture and is green in color (commonly are commonly referred to as Michigan greenstone (aka: a drab gray-green) due to the presence of some combination greenstone and Isle Royale greenstone) and are also Michi- of chlorite, , epidote, zoisite, prehnite, or pumpel- gan’s official state gemstone. The individual stellate masses lyite (fig. 3). This mineral assemblage, along with , are often chatoyant: thus the name chlorastrolite, meaning sphene, and in some cases a carbonate mineral, can partially “green star stone” (Heinrich and Robinson 2004). to completely replace the original magmatic minerals during In New Zealand, greenstone is used to describe nephrite- . jade, which is found on the Southern Island in the Tara- Although they are not normally qualified by the use of 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 mine, Mariposa County, California, Cook and Gressman (2008) mention greenstone; however, the greenstone to which they refer is, Figure 2. New a third, and certainly the most widely used, example of a Zealand greenstone greenstone. It is a very fine-grained, altered, or metamor- (nephrite-jade) pendant, tradi- tional fishhook Dr. John Rakovan, an executive editor of Rocks & Minerals, design (Maori: hei- matau). Fashioned is a professor of mineralogy and at Miami in Christchurch, University in Oxford, Ohio. New Zealand.

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553-555 WordttWise ND08.indd 553 10/6/08 9:01:44 PM tural features and their relationships to surrounding rocks, have been interpreted as being the result of changes in the dynamics of plate 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 veins or disseminated in adjacent altered rock. The deposits of the Golden Mile in Kalgoorlie, Western , are a classic example of –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 on Earth. In the Barberton, most of the gold deposits lie within facies metamorphic rocks, and it has been sug- gested that hydrothermal emplacement of the gold is related to the intrusion of and syenites that are associated Figure 3. Greenstone erratic, 3 feet across, from Oxford, Ohio. with the belt (Foster and Piper 1993). Light-colored rounded areas are . Given the preponderance of green in the natural world, including the mineral kingdom, it comes as no surprise that metamorphic, to avoid confusion I will use this adjective the term greenstone has found common usage in describ- to describe the third usage of greenstone. Metamorphic ing different green stones. As pointed out by Dr. Bill Cor- greenstones occur worldwide in rocks of essentially all ages. dua, University of Wisconsin–River Falls, an amusing cir- Best (1982) speculates that most metamorphic greenstones cumstance of nomenclature and geology is that Michigan lie buried beneath the ocean floors, where they were formed greenstone (masses of chlorastrolite) is sometimes found by metamorphism of the , made up of basalts in basalts that have been metasomatized into metamorphic and deeper crystallized . The grade (pressure-tem- greenstones. In other words, we can find greenstones in perature regime) of alteration in metamorphic greenstone is greenstone. low, typically falling within what is known as the greenschist facies (Best 1982). Because of the low grade of metamor- AKNOWLEDGMENTS phism, relict magmatic fabrics, such as pillow structures, are I thank Kendall Hauer and Peter Modreski for their reviews of this article. often preserved. Metamorphic greenstones also comprise, to varying REFERENCES degrees, large rock sequences of age (Archaean Bateman, R., and F. P. Bierlein. 2007. On Kalgoorlie (Australia), through ) that are called greenstone belts. Such Timmins-Porcupine (), and factors in intense gold min- belts also include metamorphosed sedimentary rocks and eralization. Geology Reviews 32:187–206. are most often associated with granites and . It is Beck, R. 1991. Jade in the South Pacific. In Jade, ed. R. Keverne, generally agreed that these assemblages formed at ancient 222–57. New York: Van Nostrand Reinhold. Best, M. G. 1982. Igneous and metamorphic petrology. New York: plate boundaries including oceanic spreading zones, such W. H. Freeman and Company. as today’s Mid-Atlantic Ridge, and island arcs, such as the Cook, R. B., and T. M. Gressman. 2008. The Mockingbird gold mine, Mariana Islands in the Western Pacific (De Wit and Ash- Mariposa County, California. Rocks & Minerals 83:392–401. wal 1997). Examples include the Barberton greenstone belt De Wit, M., and L. D. Ashwal, eds. 1997. Greenstone belts. Oxford (), the Abitibi and Temagami greenstone belts monograph on geology and geophysics 35. Oxford: Clarendon Press. (Ontario, Canada), the (southwestern Foster, R. P., and D. P. Piper. 1993. Archaean lode gold deposits in ), the South Pass greenstone belt (Wind River Africa: Crustal setting, metallogenesis and cratonization. Ore Mountains, Wyoming), and the Marquette greenstone belt Geology Reviews 8:303–47. (Upper Peninsula, Michigan). Heinrich, E. W., and G. W. Robinson. 2004. Mineralogy of Michigan. Greenstone belts are significant for several reasons, not 2nd ed. Houghton: A. E. Seaman Mineral Museum, Michigan Technological University. the least of which is that they are a window into the crustal Herrington, R. J., D. M. Evans, and D. L. Buchanman. 1997. Metal- evolution and the plate-tectonic history of early Earth. They logenic aspects. In Greenstone belts, ed. M. De Wit and L. D. are also host to significant metal deposits including , Ashwal, 176–219. Oxford monograph on geology and geophysics iron, and gold (Herrington, Evans, and Buchanan 1997). 35. Oxford: Clarendon Press. Differences in the nature of greenstone belts that formed McLintock, A. H., ed. 1966. An Encyclopaedia of New Zealand. Te Ara—The Encyclopedia of New Zealand, updated 18-Sep-2007. during the Archaean (3.8–2.5 billion years ago), Proterozoic URL: http://www.TeAra.govt.nz/1966/G/Greenstone/en. (2.5 billion–543 million years ago), and (543 Rakovan, J. 2005. Word to the Wise: Amygdule. Rocks & Minerals million years ago to present), especially in terms of struc- 80:287. q

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