Limitations in using the Cu isotopic composition of minerals from ancient copper mines for archaeometric purposes – a case study

Abstract

High-precision Cu isotopic analysis has been suggested as a complementary tool to Pb isotopic analysis in archaeometry. Pronounced Cu isotope ratio variations can be found in Cu-dominated minerals, stemming from the complex redox chemistry underlying their formation. In this work, the Cu isotopic composition of Cu ores (azurite/malachite) from three important mining centres exploited during the Copper and Early Bronze Age in the Northwest of Spain was evaluated using multi-collector inductively coupled plasma-mass spectrometry. The δ⁶⁵Cu data showed high variability for samples from within the same mines, with an important overlap of the ranges for the different mines. This variability was due to the heterogeneity of the copper isotopic composition observed even within a single sample from one mine divided into 13 different subsamples. The δ⁶⁵Cu values obtained for samples from the 4 mines varied within a large range (of about 4‰). These results were compared to previously published δ⁶⁵Cu data for both primary and secondary minerals and good agreement between the ranges observed was established. The comparison of δ⁶⁵Cu data for the minerals investigated in this work with 27 previously measured archaeological artefacts (arsenical copper from the Copper Age or Early Bronze Age) did not provide meaningful results, although most of these archaeological samples were previously assigned to one of the mines studied based on their clearly radiogenic Pb isotopic signature. This case study demonstrates that the potential application of the ore Cu isotopic composition for archaeological studies is strongly jeopardized by intra-mine variability.