The 19-ton mass of native copper, exhibited in the Copper Pavilion, was recovered from the bottomlands of Lake Superior, offshore between Eagle River and Eagle Harbor in Michigan's Keweenaw Peninsula. It holds a Guinness World Record. This mass of Lake Copper was discovered in 1991 by local divers Bob Barron and Don Kauppi and recovered in 2001. It is on permanent loan from the State of Michigan, Department of Natural Resources. The native copper was precipitated in a near vertical tabular vein about 1 billion years ago. The mass was horizontal when it was found as it had fallen over. The top surface of the mass is smooth because of abrasion by glacial action; the glaciers left about 10,000 years ago. The bottom surface of the Lake Copper mass is rough because it was lying on the lake bottom and thereby protected from abrasion. The surface oof the Lake Copper is coated by secondary copper minerals formed by chemical reactions with lake water. The smaller 250-pound smoothed mass of native copper on exhibit adjacent to the top of the 19-ton mass was found nearby and is of similar geologic history. Neither mass was glacially transported and they are not "float" copper.
Read more about the discovery and geology of the Lake Copper.
A small mass of “float” copper is on exhibit in the Copper Pavilion behind the 19-ton Lake Copper. Float copper refers to glacial-fluvial (river) transported native copper. Glacial ice plucked native copper from the bedrock and carried it along with many different sized rocks ranging from boulders to tiny particles. These masses of native copper were "floating" in the ice. The abrasion by rocks, smoothed and rounded the ntaive copper masses as well as largely removed attached minerals. Melting of the glaciers, 10,000 years ago, deposited the float copper and other rocks on the surface. Chemical reastions between the native copper and surface waters resulted in new copper minerals coating the surface of the float copper such as green-colored malachite. Note that the float copper is smooth on both sides as compared to the 19-ton Lake Copper which is rough on the back because it was not carried by the glacier and thus, the glacial processes only smoothed the top side.
Read more about Float Copper Geology.
While the 19-ton Lake Copper on exhibit in the Copper Pavilion is impressive, single masses of native copper weighing up to 400 tons were found underground in the mines.