Geochemical Analyses Of Native Copper In North America: Implication For Archaeological Provenance Studies

Native copper is one of earliest metals used by human, it is an important material related to early civilization. There are the richest native copper deposits in North America, 14C dating indicates that native cooper had been utilized by American indigenous people in North America since 6800BP. The use continued until 17th century when European smelted copper entered North America. Archaeological studies show that native copper is the only material made into copper artifacts. For thousands of years, Indians fabricated a large number of copper artifacts by hammering, which were discovered in different prehistoric sites in North America. Before Columbus landed the America, North America was still in a primitive social stage. The life of aboriginals in North America could be only inferred through archaeological researches. Exploring the provenance of raw materials of those copper artifacts is an important content of North America prehistory. Understanding where native copper originated can provide critical information regarding trading routes, the interaction of cultures, the exploitation and use of mines.To establish the geochemical signatures of native copper of North America, three techniques: trace element, lead isotope and copper isotope analyses have been employed in this study. These three geochemical techniques are all important approaches to determine the geologic sources of artifact copper. Trace element analysis is a common method and has a history over one and half hundred years used to provenance study in North America. In this study, Pb, one of trace elements of native copper has been analyzed and its application to provenance study has been discussed. For the first time, the copper isotope technique is applied to archaeological provenance study of native copper in North America in this study. The analysis of lead isotope for native copper is also a pilot study in North America archaeology. Prior to experimental analysis, the history of mining of native copper and the copper culture in North America have been introduced and discussed. In order to understand the provenance study of native copper in North America better, reviews and discussion for three geochemical techniques have been made before the analyses.The main results and conclusions of this study are as following:Firstly, the Pb concentration of 66 native copper samples from several regions in North America has been determined by graphite furnace atomic absorption spectrometry (GF-AAS) and inductively coupled plasma mass spectrometer(ICP-MS). The results show that the native copper in North America has very low Pb content and exists obvious heterogeneity of Pb even in a single ore sample. By comparing GF-AAS and ICP-MS, we found different analytical techniques can yield different results. This indicates that care should be taken when choosing analytical technique in trace element study. According to the results of this study and previous analyses, it is found that Pb concentration may be used to distinguish native copper in North America and historical European smelted copper. Meanwhile, the analysis results also remind researchers need to pay special attention to Pb concentration and its heterogeneity when they are applying Pb isotope and trace element analysis to archaeology. The composition of 24 native copper samples (including 5 smelted native copper samples) has been analyzed by scanning electron microscope energy dispersive spectrometer (SEM-EDS). The results show that native copper is an extremely pure metal, generally purer than 99%.Secondly, the Pb isotope composition of 13 native copper samples from 4 regions has been measured by thermal-ionization mass spectrometry (TIMS). The result indicates there is no high-radiogenic Pb found in all samples. The Pb isotope compositiom of samples from Minong of Michigan, Bisbee of Arizona and Chititu of Alaska are different each other. Significant difference of Pb isotope signature also exists between these three regions and Keweenaw peninsula, Michigan where most quantity of native copper were mined by prehistoric Indians. The result of study suggests that Pb isotope can be used as a fingerprint to trace the geological sources of artifact copper in North America.Finally, the copper isotope signature of 15 samples from 5 native copper regions has been determined by multiple-collector plasma-source mass spectrometry (MC-ICP-MS). Results reveal a relative small range of variation (δ65Cu) of copper isotope within these samples. Along with previous analyses, we found that copper isotope composition of primary native copper is significantly different from secondary native copper. A relatively abundant data show that the primary native copper from Michigan has a relatively homogenous copper isotope composition and seems distinct from the other regions. In addition, significant different copper isotope variation exists between primary native copper in North America and chalcopyrite and chalcocite. In general, the latter two copper minerals are major sources for smelted copper. Conclusion can be drawn that copper isotope can be used as tracer in archaeological provenance studies of native copper. It will be a helpful tool in distinguishing native copper from different regions in North America as well as native copper of North America and European smelted copper.