Greenstone - Miami University

Word to the Wise
JOHN RAKOVAN
Department of Geology
Miami University
Oxford, Ohio 45056
[email protected]
I
Greenstone
ron is the most abundant chromophore (an element that
imparts color to a mineral by its presence in the structure) in the earth. When iron is in its divalent oxidation
state (Fe2+), as is found in such minerals as olivine, pyroxenes, 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 minerals, 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 Michigan’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 nephritejade, which is found on the Southern Island in the Taramakau-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 metamorDr. John Rakovan, an executive editor of Rocks & Minerals,
is a professor of mineralogy and geochemistry at Miami
University in Oxford, Ohio.
Figure 1. Michigan greenstone (chlorastrolite) from Isle Royale.
The amygdule width is 2.5 cm. Seaman Mineral Museum specimen, John Jaszczak photo.
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 pumpellyite (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
Figure 2. New
Zealand greenstone
(nephrite-jade)
pendant, traditional fishhook
design (Maori: heimatau). Fashioned
in Christchurch,
New Zealand.
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Figure 3. Greenstone erratic, 3 feet across, from Oxford, Ohio.
Light-colored rounded areas are felsic xenoliths.
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-temperature 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 metamorphism, 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 Ashwal 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 struc554
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 suggested 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 describing different green stones. As pointed out by Dr. Bill Cordua, University of Wisconsin–River Falls, an amusing circumstance 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.
AKNOWLEDGMENTS
I thank Kendall Hauer and Peter Modreski for their reviews of
this article.
REFERENCES
Bateman, R., and F. P. Bierlein. 2007. On Kalgoorlie (Australia),
Timmins-Porcupine (Canada), and factors in intense gold mineralization. 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:
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Cook, R. B., and T. M. Gressman. 2008. The Mockingbird gold mine,
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De Wit, M., and L. D. Ashwal, eds. 1997. Greenstone belts. Oxford
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Herrington, R. J., D. M. Evans, and D. L. Buchanman. 1997. Metallogenic aspects. In Greenstone belts, ed. M. De Wit and L. D.
Ashwal, 176–219. Oxford monograph on geology and geophysics
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URL: http://www.TeAra.govt.nz/1966/G/Greenstone/en.
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