"Rock of the Month # 106, posted April 2010" --- This sample has a striking heft, and probably contains more of the hackly, brightly metallic native copper than meets the eye. The host is a bright green altered volcanic rock, rich in green sheet silicates (chlorite, clay minerals) and lesser green, granular epidote. The pearly, white to mauve "pyramid" atop the sample is actually a section of a thin veinlet of the calcium borosilicate datolite, which cuts the host rock. The native copper has been treated to remove surface tarnish and artificially enhance its natural colour and lustre. The Caledonia mine which furnished this specimen has been worked as a specimen mine in recent years. Sample 1656, purchased in 1995.

Native copper and datolite, rather unusual minerals, are noted species in parts of the Keweenawan (circa 1100 million-year-old) Midcontinent Rift of North America. The bulk rock is rich in iron and calcium, copper and boron. Datolite is a monoclinic borosilicate, CaBSiO₄(OH). Other minerals found in the local basic lava flows, including the "amygdaloid lodes" are calcite, zeolites, native silver and copper arsenides.

The native peoples of the Great Lakes region knew of surface occurrences, including large boulders, of metallic copper, and recovered and worked this natural treasure, preparing a range of tools, weapons and ornamental goods (e.g., Wilson, 1856; Whittlesey, 1862; Drier and Du Temple, 1961). Radiocarbon dating of the charcoal residues in the tang of a projectile point from northern Minnesota confirmed an age of almost 7,000 years before present (Beukens et al., 1992), confirming the great antiquity of this ancient art. The second half of the nineteenth century saw a new industrial era of copper mining, mostly using modern underground techniques, employing the skills and sweat of a new breed of miner. Many of these workers came from Cornwall and elsewhere in Europe (Rickard, 1905; Wilson and Dyl, 1992).

The geology of the copper country was soon surveyed in detail, and many mines opened, especially on and adjacent to the Keweenaw peninsula which juts northeastwards into Lake Superior (Irving, 1883; Bornhorst, 1992; Wilson and Dyl, 1992). The Caledonia mine and many other old mining sites have yielded a trove of mineral specimens, valued around the world (Heinrich, 1976; Bornhorst and Whiteman, 1995; Rosemeyer, 1997). For a tremendous display of the region's mineral wealth and mining history, be sure to visit the "A.E. Seaman Mineral Museum" on the Michigan Tech campus in historic Houghton!

References

Beukens,RP, Pavlish,LA, Hancock,RGV, Farquhar,RM, Wilson,GC, Julig,P and Ross,W (1992) Radiocarbon dating of copper-preserved organics. Radiocarbon 34, 890-897.

Bornhorst,TJ (editor) (1992) Keweenawan Copper Deposits of Western Upper Michigan. SEG Guidebook 13, 197pp.

Bornhorst,TJ and Whiteman,RC (1995) Native copper and associated minerals in basalts at the Caledonia mine, western Upper Michigan. Abs. 41st Annual Meeting, Institute on Lake Superior Geology, vol.41 part 1, 80pp., 3-4, Marathon, Ontario.

Drier,RW and Du Temple,OJ (editors) (1961) Prehistoric Copper Mining in the Lake Superior Region. R.W.Drier, Calumet, MI, 222pp.

Heinrich,EW (1976) The Mineralogy of Michigan. Michigan Department of Natural Resources, Geological Survey Division, Bull. 6, 225pp.

Irving,RD (1883) The Copper-Bearing Rocks of Lake Superior. USGS Monograph 5, 464ppp. plus 29 plates.

Rickard,TA (1905) The Copper Mines of Lake Superior. Engineering and Mining Journal, New York and London, 164pp.

Rosemeyer,T (1997) Mineralogy of the Caledonia mine, Ontonagon County, Michigan. Mineral.Record 28, 52.

Whittlesey,C (1862) Ancient Mining on the Shores of Lake Superior. Smithsonian Contributions to Knowledge 155, 29pp.

Wilson,D (1856) The ancient miners of Lake Superior. Canadian Journal of Industry, Science, and Art, New Series No.3, The Canadian Institute, Toronto, 225-237.

Wilson,ML and Dyl,SJ (1992) The Michigan copper country. Mineral.Record 23 no.2, 1-72.