Natural Science in Archaeology

7.4 Native Copper (Cu) 155
Native copper is mined from lode deposits; from river, shoreline, and lag deposits
of rounded nuggets; and from glacial till. Subject only to minor surface alteration,
native copper is nearly indestructible in the surface geologic environment.
In Precolumbian North America, most of the native copper available at or near
the surface was derived from mafic igneous rocks. North of the Rio Grande River,
prehistoric people did not smelt, melt, cast, or alloy metals, relying instead on the
abundant native copper. Large deposits of native copper occur in the basaltic lavas
and related sedimentary rocks of the Lake Superior region; these are by far the
most extensive native copper deposits in the world. Other primary deposits in North
America include those of the Appalachian belt, and in southeast Alaska, the Yukon
Territory (especially the Coppermine River area), British Columbia (especially
Victoria Island), the Northwest Territories, Labrador, and Cap d’Or, Nova Scotia.
Masses and sheets of native copper in the Keweenaw Peninsula of Michigan
occur at the surface throughout a zone 5 km wide and nearly 150 km long. Outcrops
are also common on Isle Royale and Michipicoten Island in Lake Superior. In prehistoric
times, nuggets in a wide range of sizes must have been abundant in streams
and along the Michigan shores of Lake Superior. Supplies available at the surface
did not always exceed demand, however, as shown by the thousands of shallow pits
dug to recover near-surface pockets of metallic copper. During the Archaic Period,
inhabitants of the western Great Lakes area made a broad range of native copper
implements (Fig. 7.6).
Fig. 7.6 Archaic copper
tools from the Lake Superior
region, USA