RE-MO-CU-OS SULFIDE FROM THE EKOJOKI NI-CU DEPOSIT, SW FINLAND

This short communication reports the discovery and composition of the first Os-bearing rhenium sulphide. It occurs as a single euhedral crys tal in the early Proterozoic Ekojoki Ni-Cu deposit in Finland, being e nclosed by interstitial magmatic Fe-Ni sulphides. Electron microprobe analyses are most consistent with a general formula of Cu(Re,Os,Mo)(5) S-9. The Re-Os model age calculations indicate that approximately 2/3 osmium is ''common'' Os that was accommodated by the sulphide at the t ime of crystallisation, 1/3 being produced via post-crystallisation de cay of Re-187 to Os-187. We propose that this sulphide crystallised fr om an immiscible sulphide liquid that was unusually rich in rhenium du e to extensive assimilation of black schist sulphides by the magma.