Challenges of using copper isotope ratios to trace the origin of native copper artifacts: An example from the Keweenaw Peninsula

In an effort to understand how and if Cu isotopes can be used to trace native copper artifacts to their mineral deposits of origin, this study presents Cu isotope measurements from weathered native Cu artifacts and ores known to be derived from the Precambrian native copper deposits of Michigan. The five weathered artifacts have Cu isotope compositions ranging from δ⁶⁵Cu= +0.54 to -1.15%. Weathered glacial till native copper nuggets range from δ⁶⁵Cu= -0.12 to +0.54%, non-weathered ores have δ⁶⁵Cu= +0.33 ± 0.2% (n= 42 from the literature and this study), a completely oxidized copper rind derived of large glacial boulder of native copper has -0.04%. The oxidized rinds along with the weathered artifacts possess isotopically lighter signatures in comparison to the non-weathered ores and interiors of weathered copper nuggets. The copper isotope data indicate the interiors of oxidized nuggets correlate with the non-weathered ores. Copper from the artifacts was sampled as micro drill bits (0.001-0.0009g) and larger cut pieces of the artifacts (>0.5g). Only the larger sample artifacts have the same copper isotope composition as the non-weathered ores, and not the oxidized rinds, and non-weathered interiors of copper nuggets. Therefore, when considering the unreacted interior of the native copper artifacts, the copper isotopic composition matches that of the known copper ore source. In contrast, weathering clearly depletes ⁶⁵Cu on the surfaces of artifacts and micro-sampling of the outer rims does not yield similar isotope results between sources and artifacts.