Characterisation of the raw metal sources used for the production of copper and copper-based objects with copper isotopes |
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Authors: | Sabine Klein Gerhard Peter Brey Soodabeh Durali-Müller Yann Lahaye |
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Institution: | 1.Institut für Geowissenschaften, Petrologie und Geochemie,J.W. Goethe Universit?t,Frankfurt a. M.,Germany;2.Research and Development XRF,Bruker AXS GmbH,Karlsruhe,Germany;3.GTK-Geological Survey of Finland,Espoo,Finland |
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Abstract: | Copper isotopes can be successfully used to determine the origin of copper or bronze artefacts from either primary or supergene
sulphide or hydrocarbonate ores. In conjunction with lead isotopes, they provide information on the origin and type of the
metal ore. We demonstrate this in this paper from the combination of literature and own data on metal ores and artefacts (coins
and ingots). Low-temperature hydrocarbonates (esp. malachite and azurite) do not fractionate the copper isotopes amongst each
other and have identical lead isotopes. Substantial fractionation of copper isotopes, however, occurs between copper sulphides
and hydrocarbonates (malachite, azurite) such that the 65Cu isotope is always enriched in the oxidised relative to the sulphide phase with a clear distinction between the two ore
types. Expressed in the δ65Cu notation, we assigned supergene sulphides to values less than −0.4‰ down to negative values of −2‰ and more, primary sulphides
to a range between −0.4 and +0.3‰ and hydrocarbonates to positive values higher than +0.3‰. We have applied these boundaries
to copper coins and ingots from the time of the Roman emperors with known ages from Augustus up to 250 AD. The deposit fields
of the metal used for the production of the coins were previously identified from the lead isotope ratios to lie in the Southwest
and the Central South of Spain. From the combination of the lead and copper isotopes and the exact time constrains, we could
develop a picture of the change in mining activities in Spain involving continued mining sulphide ore deposits and, indicated
by positive δ65Cu values as proxies for malachite and azurite, the opening of new mines in various time slots. This first application shows
that copper isotopes will become the most important tool in archaeometallurgy to distinguish between the exploitation of deeper-seated
primary and supergene sulphide ores and shallower, secondary hydrocarbonate ores. This will become especially relevant for
archaeometric questions regarding the distinction between occasional and intentionally produced alloys. |
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