Mithras in Tróia (Portugal): Analysis of the Roman marble bas-relief with the banquet of the Gods Mithras and Hélios (3rd c CE)

Tróia is the name of a sandy peninsula on the left bank of the river Sado in the southwestern Atlantic coast of Portugal. On the north tip of the peninsula close to the estuary of the river facing the Roman city of Caetobriga (currently Setúbal) in the mainland, a Roman industrial settlement based on the production of salt-fish goods evolved since the first century CE. In the mid forth century CE an early Christian basilica with frescoes was erected in an area previously covered by salt-fish vats. Life in the settlement of Tróia would cease during the fifth/ sixth centuries CE and the place would fall into oblivion until the sixteenth century.
When archaeological excavations in Tróia started in the turn of the twentieth century, five sculpted fragments of white marble were found in an area close to the Early Christian Basilica as archaeologist Marques da Costa later described (1934). When reassembled, the fragments composed a panel with the depiction of the banquet of the gods Mithras and Helios. Although incomplete, the panel made part of a larger composition, possibly a diptych or triptych, where the ritual of the killing of the bull - the taurobolium - might have been represented as well. Since its discovery, the bas-relief of Mithras has been considered as one of the finest of its genre as immediately stated by Franz Cumont. Currently, a replica of the bas-relief of Mithras is on display at the National Museum of Archaeology in Lisbon whereas the original is in private hands.
The goal of our work is to introduce the analysis carried out to determine the provenance of the marble of the Mithraic bas-relief of Tróia. A multi-method approach combining polarized-light microscopy, cathodoluminescence, X-ray powder diffraction, and stable C and O isotope analysis was applied to identify the marble provenance. The comparison of the results with the available databases confirms the use of a statuary white marble variety from the Lusitanian Estremoz Anticline, in the Alto Alentejo.
With this procedure it is expected to obtain a valuable complementary source of information for the study of the cult of Mithras in the westernmost part of the Roman province of Lusitania, a work still to be done. Moreover, this analysis is part of a wider context of research aiming at understanding the architecture and decoration in Tróia during Antiquity.