Mineral Composition of Ores Used at Ustye-I, a Fortified Settlement of the Bronze Age (Chelyabinsk Region)

Abstract

At the fortified settlement of the Bronze Age, Ustye-I (South Trans-Urals), metal products of copper, arsenic and tin bronzes, numerous fragments of metallurgical slags, as well as fragments of copper ores have been discovered. According to the results of early studies, pure copper predominates among metal items (53.4% of finds), but also a significant part contains an admixture of arsenic (25.4% of finds), tin (13.6% of finds) and both components simultaneously (7.6% of finds). Several alloying components of the samples are simultaneously found in slags: Pb (50% of cases), As (49% of cases), Zn (40% of cases), Bi (38% of cases), Sn (7% of cases), Ni (4% of cases), Sb (1% of cases). To identify the relationship between the incoming raw materials and the received metal, 15 ore samples were examined. The ores studied are ‘gravelly’ clasts with a size of 5 mm to several centimeters, and ‘sandy’ ores with a size of 2–3 mm. According to their mineral composition, ores are subdivided into malachite, azurite, and limonite. The main copper minerals are malachite and azurite; chrysocolla is found occasionally, chalcopyrite, cuprite are rarely found. Copper is bound to Fe- and Mn-hydroxides. We have identified copper slag fragments in ‘sandy’ ores. In all types of ores, natural alloying components are very rare and are represented by Pb and Zn. These elements can form their own minerals (mottramit, a mineral of the plumbogummite family) or act as an impurity to malachite (up to 0.62 wt% ZnO), azurite (up to 1.12 ZnO and up to 2.37 PbO), iron hydroxides (up to 2.66 ZnO and up to 2.02 PbO) or manganese hydroxides (up to 0.41 ZnO and up to 9.48 PbO). No As and Sn impurities were found. The presence of numerous fragments of brown iron ore at the settlement, the close location of copper mines indicates that the settlers were engaged in the extraction and processing of copper ore. The widespread occurrence of ores, consisting mainly of malachite and azurite, indicates that the ores were mined from the upper parts of the oxidation zone, and the metal smelted from these ores must correspond to pure copper. The distribution of Pb and Zn impurities indicates that some of the copper ores originate from the oxidation zones of pyrite ores or deposits of other formational types with a similar ore composition. The difference in the compositions of the ores found at the settlement and the compositions of slag and metal is associated with several sources of ores. The sources of copper ores could be mines located near the settlement. Arsenic-bearing ores could have come from mines in adjacent areas. Tin ores were brought in as a result of trans-regional exchange with the Central Asian region.

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