GEOCHEMICAL PROSPECTING AT MT. PROSARA AND MOTAJICA, NORTHERN BOSNIA: EXAMINING POTENTIAL SITES OF BRONZE AGE PLACER TIN ORE MINING

Tin is a valuable economic resource and has been utilized since prehistory to produce the copper alloy, bronze, the defining technology of the Bronze Age. The rarity of tin deposits, along with their importance to Bronze Age cultures, makes them archaeologically noteworthy. The Balkans have been identified as a region from which tin ore was mined in the Bronze Age, with six sites identified by Durman (1997) as the most likely locations of mineralization. Placer tin ore has been confirmed at Mt. Cer and Bukulja in Serbia. The remaining four sites identified by Durman (1997) had not been groundtruthed by geochemical survey. This study examines two of these localities, the inselbergs of Prosara and Motajica that lie on the northern border of Bosnia.

To test the hypothesis that these sites may be tin-bearing, geochemical prospecting was conducted on coarse sediments from streams flowing from the major drainages of these mountains. Cassiterite (SnO₂) is hard (Mohs hardness 6-7), dense (6.9 g/cm³), and stable under surface conditions, and so accumulates as placer deposits. Accordingly, sediments were collected from sand/gravel bars, sieved, sluiced, and panned on site to prepare a rough heavy mineral separate. The heavy mineral fraction of each sample was then further concentrated in the lab using sodium polytungstate heavy liquid (2.94 g/ml). The mineral magnetite was magnetically extracted and the heavy sediments were sorted according to grain size fractions: very coarse sand (>850μ), coarse sand (850-420μ), and fine sediments (<420μ). The chemical composition of these heavy mineral separates were analyzed using an Olympus Innov-X X-ray fluorescence (XRF) hand apparatus with a tin detection limit of approximately 150 ppm.

The XRF analysis indicated high concentrations (> 1%) of elements common to heavy mineral sands (Fe, Ti, Mn, Ca, and K). However, no tin was identified. Thus, it is highly unlikely that northern Bosnia supplied tin ore for bronze production in prehistory. Further separation, such as magnetic susceptibility, and an SEM-EDS analysis is underway to verify the absence of tin in the samples and to further characterize the heavy mineral assemblages in the major streams of Prosara and Motajica.