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Above: two views of a splendid pair of bookends, sawn from a "float copper" mass of native copper,
found naturally in the vicinity of Houghton, Hancock and related historic mining towns of the Keweenaw peninsula,
which is a highlight of the spectacular Lake Superior coastline.
These bookends (sample 2895) have a combined weight of approximately
10.4 kg (almost 23 pounds), quite sufficient
to perform
their task!
Unlike the majority of my samples, these specimens work for a living.
The metal is not appreciably magnetic. The superb electrical conductivity of copper defeats my magnetic susceptibility meter. Each piece is about 12 cm high, 9-10 cm wide and 5.5 cm thick. They were purchased from the A.E. Seaman Mineral Museum (see below) in May 2010. The parent mass of copper was found in glacial drift in Houghton county. The metal appears >99% pure, with very few or no inclusions of the original host rock. The colour suggests that the metal is pure not only in terms of freedom from foreign bodies, but also in chemical terms, with only traces of other elements (e.g., iron, silver or arsenic) alloyed with the copper. The exterior of the mass displays a thin rind of bright green copper carbonate, malachite. A similar green weathering product ("verdigris") occurs on the oldest, 40-year-old Canadian one-cent piece, in the foreground!
"Rock of the Month #143, posted for May 2013" ---
Native copper is quite widespread, but only in a few
mining districts does it occur as a plentiful mineral.
I have not researched the point, but would expect that native Cu is the most
abundant metal by total mass in the crust. This despite the fact that
native Au and native Ag are widely reported: to an economic geologist, any speck of
the precious metals is worthy of note!
But Cu is a base metal, and other base metals are uncommon (native Fe)
to rare (native Pb, native Bi) in nature, if they occur at all.
Other elements that occur in native forms include S, As, Te, Hg and
platinum-group elements (PGE) such as Pt.
To recap, native gold is doubtless reported more often than
native copper, but generally (of course) in comparatively small quantities.
Copper is a cubic metal in the gold group, and as such occurs in the
first set of minerals in Dana's classical "System of Mineralogy" (Palache
et al., 1944). Copper (mineral species 1114 in Dana's System)
is found in the weathered, oxidized "supergene" zone of
many mineral deposits. It may be alloyed with arsenic to form
other minerals found in the Keweenaw, and may be intergrown with
native silver.
The famous copper mines of the Keweenaw district
are described in many publications
(see, e.g., Bornhorst, 1992; Bornhorst and Rose, 1994;
Rosemeyer, 2011; and a host of local booklets describing individual mines).
The deposits are spatially associated with voluminous
basaltic lavas of the Proterozoic, Keweenawan Midcontinent Rift
of North America. This failed rift system generated epic quantities
of basalts and related intrusive igneous rocks.
Some of the intrusions in the Rift are enriched in Cu, Ni and PGE,
but the association of basalts and predominantly clastic sediments
(sandstone, conglomerate, shale)
in the
Keweenaw district are for the most part enriched in Cu and locally in Ag.
The masses
of "float copper" found on and near surface, and then
underground in the great mines of the district,
could weigh tonnes - exceptionally a few hundred tonnes - each!
The copper was first discovered thousands of years before the
mining heyday of the 19th century: native Americans
worked, crafted and traded copper widely.
When in Houghton,
be sure to visit the
"A.E. Seaman Mineral Museum"
on the Michigan Tech campus!
First opened as a museum in 1902, the modern collections
moved into new premises and reopened on 04 July 2011.
The museum is described elsewhere (e.g., Wilson and Dyl, 1992;
Dyl, 1995), as are some of the enthusiasts whose
careers and collecting passions
contributed to the remarkable museum
we see today (e.g.,
Rosemeyer and Dyl, 2000; Lininger, 2001).
The latest history, beautifully recounted and
illustrated with specimens from some key acquisitions,
was prepared by Robinson and Rann (2012).
References
Bornhorst,TJ (editor) (1992) Keweenawan Copper Deposits of
Western Upper Michigan. SEG Guidebook 13, 197pp.
Bornhorst,TJ and Rose,WI (1994) Self-Guided Geological Field
Trip to the Keweenaw Peninsula, Michigan. Institute on Lake
Superior Geology, vol.40, part 2, 185pp.
Dyl,SJ (1995) Seaman Mineral Museum. Mineral.Assoc.Canada Newsletter 53,
10-13.
Lininger,JL (2001) The life and times of museum builder A.E.
Seaman. Matrix, a Journal of the History of Minerals 9 no.4,
143-159.
Palache,C, Berman,H and Frondel,C (1944) The System of
Mineralogy of James Dwight Dana and Edward Salisbury Dana.
Volume 1, Elements, Sulfides, Sulfosalts, Oxides. John Wiley and
Sons, Inc., New York. 7th edition, revised and enlarged, 834pp.,
pages 89,99-102.
Robinson,GW and Rann,R (2012) The A.E. Seaman Mineral Museum, its History
and Collections. A.E. Seaman Mineral Museum, Michigan Technological
University, Houghton, MI, 95pp. plus CD-ROM.
Rosemeyer,T (2011) Michigan's Copper Country. Rocks &
Minerals 86 no.3, 206-227.
Rosemeyer,T and Dyl,SJ (2000) John Thorley Reeder: gentleman
collector of the Michigan copper country. Matrix, a Journal of the
History of Minerals 8 no.4, 159-173.
Wilson,WE and Dyl,SJ (1992) The Seaman mineral museum.
Mineral.Record 23 no.2, 73-76.
MUSEUM MOMENT #5