Natural Science in Archaeology

156 7 Metals and Related Minerals and Ores
Native copper also occurs widely as a secondary alteration mineral within the
oxidized zones of copper sulfide deposits. Strong oxidation occurs in these
deposits because the abundant pyrite dissolves in water to form sulfuric acid and
ferric sulfate. This oxidized zone is always close to the ground surface and is
often visible at the surface. In the sulfide copper ores of western North America,
native copper is a common but minor constituent of the oxidized zone. One of the
largest native copper deposits of this type is located in the Santa Rita district of
New Mexico where millions of pounds of native copper were mined in the early
days of mining.
The copper deposits of the southwestern United States have been important economically
as large low-grade copper sulfide deposits. The oxidized zones near the
surface sometimes include native copper, but little has been published about the
native copper occurrences and their accessibility at the surface. In Arizona, native
copper occurs in the enriched zone at San Manuel, and native copper has been
found in most of the deposits of the Banner district. Anthony et al. (1995) detail the
many known occurrences of native copper in Arizona. They consider it to be “very
abundant in Arizona, usually in small amounts in the oxidized portions of copper
deposits”. Bullock (1981) lists the native copper occurrences in Utah by county and
by mine or mining district.
A sourcing study of North American native copper artifacts showed that arsenic
is the best element to use for distinguishing between primary native copper from
mafic volcanic rocks and secondary native copper from oxidized copper sulfide
deposits (Rapp et al. 2000). In this study, the secondary native copper contained
much lower concentrations of arsenic. Native copper is widely, but not abundantly,
found in the oxide zones of copper sulfide deposits in Mexico (Panczner 1987).
Despite the information available from the geologic literature about copper deposits
within reach of Aegean Bronze Age metalsmiths by direct exploitation or trade, we
have insufficient data on these deposits to understand their chemical and mineral
composition (for sourcing) or their size and surface expression (for assessing their
availability for Bronze Age exploitation).
Masses of “float copper” transported and deposited by glacial action have been
found in the glacial drift in Michigan, Wisconsin, Iowa, Illinois, Indiana, Ohio,
New York, and Minnesota. When discovered, such loose nuggets undoubtedly were
fashioned into artifacts by copper-using cultures.
Watson (1923) has detailed the occurrence of native copper in the Appalachian
region in basaltic flows. The main Blue Ridge outcrop belt extends from central
Virginia through Maryland into Pennsylvania (South Mountain). A second belt
occurs in the piedmont of Virginia and North Carolina. Native copper is the chief
ore mineral of both belts. Copper mining in the state of Maryland began before the
Revolutionary War (Parre 1964).
In southeastern Alaska, native copper occurs in placers in Chititu Creek, a tributary
of the Nizine River about nine miles southeast of McCarthy. Copper nuggets
are still quite abundant in the stream gravels. The native copper in this region weathered
out of the Nizine Greenstone belt, and is found in many streams in addition
to Chititu Creek. An almost unlimited copper supply would have been available to