Roman lead silver smelting at Rio Tinto: the case study of Corta Lago

Abstract

The Rio Tinto area is famous for the presence there of a rich concentration of several metals, in particular copper, silver and manganese, which were exploited from the Bronze Age up to few decades ago. The modern mining industry has been responsible for both bringing to light and destroying signs of past exploitation of the mines and metal production there. \nThe Corta Lago site owes its discovery to the open cast exploitation that reduced the whole mount of Cerro Colorado to an artificial canyon. This exploitation left behind sections of antique metallurgical debris as well as revealing the old underground workings. The Corta Lago site dates from the Bronze Age up to the 2nd century AD, consisting mainly of silver and copper production slag, but also including litharge cakes, tuyéres and pottery. \nThe project focused on the study of silver production slag from different periods using petrograhical and chemical techniques, such as Optical Microscopy, X-Ray Diffraction, X-Ray Fluorescence, Scanning Electron Microscopy associated to Energy Dispersive Spectrometry and Multi-Collector Inductively Coupled Plasma Mass Spectrometry. The aim of the project was to reconstruct the metallurgical processes of the different periods, detecting any differences and similarities. \nThe mineral exploited was jarosite, XFe3+3(OH)6(SO4)2, where X can be K, Na, Pb, Ag and NH4, and the results show that the system of production was much more similar to iron production than silver. In the slag, the main mineral is fayalite, and the concentration of lead is around 1%. \nThese results and study of the jarosite suggest the possibility of different sources of lead for the collection of silver in the system, and this is the reason for the utilization of the MC-ICP MS for the analysis of the lead isotopes. The results for the isotopes indicate the addition of a second source of lead used as lead metal in the system to increase the amount of lead and improve the collection of silver. \nThe differences in the processes used at different periods are the amount of lead coming from another site that was added, and the level of standardization of the system. While the first difference is evident in a comparison between the pre-Roman process and one of the Republican phases, the second is mainly visible between the pre-Roman and Roman processes. At this stage the aim of the project was to attempt to correlate the differences in the processes, metallurgical skills and geological knowledge.

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