Word to the Wise JOHN RAKOVAN Department of Geology Miami University Oxford, Ohio 45056 [email protected] Greenstone ron 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 I 2+ state (Fe ), 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. Figure 1. Michigan greenstone (chlorastrolite) from Isle Royale. The amygdule width is 2.5 cm. Seaman Mineral Museum speci- In vesicular basalts from the Lake Superior region (rocks men, John Jaszczak photo. of the Keweenawan System), a variety of pumpellyite called chlorastrolite forms amygdules (Rakovan 2005) in compact, phosed mafic igneous rock (e.g., basalt, gabbro, or diabase) 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, actinolite, epidote, zoisite, prehnite, or pumpel- gan’s official state gemstone. The individual stellate masses lyite (fig. 3). This mineral assemblage, along with albite, 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- metamorphism. 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 gold 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 geochemistry at Miami in Christchurch, University in Oxford, Ohio. New Zealand. Volume 83, November/December 2008 553 553-555 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 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 Figure 3. Greenstone erratic, 3 feet across, from Oxford, Ohio. with the belt (Foster and Piper 1993). Light-colored rounded areas are felsic xenoliths. 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 oceanic crust, made up of basalts in basalts that have been metasomatized into metamorphic and deeper crystallized gabbros. 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 Precambrian age (Archaean Bateman, R., and F. P. Bierlein. 2007. On Kalgoorlie (Australia), through Proterozoic) that are called greenstone belts. Such Timmins-Porcupine (Canada), and factors in intense gold min- belts also include metamorphosed sedimentary rocks and eralization. Ore Geology Reviews 32:187–206. are most often associated with granites and gneisses. 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 (South Africa), the Abitibi and Temagami greenstone belts monograph on geology and geophysics 35. Oxford: Clarendon Press. (Ontario, Canada), the Isua greenstone belt (southwestern Foster, R. P., and D. P. Piper. 1993. Archaean lode gold deposits in Greenland), 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 copper, 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 Phanerozoic (543 Rakovan, J. 2005. Word to the Wise: Amygdule. Rocks & Minerals million years ago to present), especially in terms of struc- 80:287. q 554 ROCKS & MINERALS 553-555 WordttWise ND08.indd 554 10/6/08 9:01:45 PM.