Bronze Age metallurgical slags from the South Urals: types, mineralogy and copper sources

In the paper, an investigation of copper metallurgical slags found in the South Urals and dating to the Bronze Age between 4,000 and 1,300 BCE is presented. Four main mineralogical types are distinguished according to their mineral composition: Cr-rich-spinel-containing olivine, sulphide-containing olivine, sulphide-containing glassy and pyroxene type. The first slag type is composed of zoned olivine, magnetite and glass having relicts of Cr-rich spinels and serpentinites that indicate the use of azurite-malachite ores from ultrabasic rocks. These finds, determined in sites of the Early Yamna period in the Cis-Urals, are widespread in Sintashta culture. The second type, composed of skeletal olivine crystals, magnetite and wustite with copper sulphide droplets, belongs to the Alakul period. Its raw material sources consist of sulphide ores from a secondary enrichment zone of volcanogenic massive sulphide (VMS), skarn and Cu-porphyry deposits. The sulphide-containing glassy type, primarily composed of glass with copper and sulphide droplets, is obtained from slags recovered in Srubna cultural sites. Here, the raw material sources were rich sulphide ores of cupriferous sandstones. The pyroxene type is composed of augite, pigeonite and/or wollastonite with a small amount of glass and sulphide inclusions. According to the composition of glass, slags can be distinguished into the Cis-Uralian type, including cupriferous sandstones, and the Trans-Uralian type having various volcanogenic-hydrothermal deposits as raw materials. Essential Ca, P, Ba and REE impurities in slag glass demonstrate the use of barite, calcite and bone fluxes. The phase diagrams, crystallisation temperatures and experimental data revealed that slag melt temperatures ranged between 1150-1300 degrees C.