Platinum and gold mineralization in the Yubdo mafic-ultramafic rocks, Western Ethiopia: Historical perspective and some new results

摘要

The Yubdo mafic-ultramafic rocks have been known for their platinum and gold mineralization since ancient times. These mafic-ultramafic rocks are bounded on both sides by low-grade volcano-sedimentary sequences of the Arabian-Nubian Shield (ANS). Further to the east and west, rocks metamorphosed to amphibolite facies occur, which are thought to be an extension of the Mozambique Belt.Platinum has been mined in the Yubdo area since 1926 by various private and government companies. A total of 2700 kg Pt has been mined and an approximate resource of 20 tons Pt has been inferred to be still present in the lateritic horizon, with an average grade of 0.4 g/m~3. This work summarizes the mining and exploration activities conducted in the area, together with the recent findings of primary PGM in drill core samples. Based on the chemical composition of Platinum Group Minerals (PGM) and the discovery of PGM inclusions in Pt-Fe placer nuggets, we suggest a two-stage model for the formation of the Pt-Fe nuggets: a) magmatic PGM formation; b) later overprint involving hydrous/ hydrothermal fluids resulting in the remobilization and concentration of PGE and gold in the Yubdo mafic-ultramafic body The chondrite-normalized plots of PGE abundances and the chromite chemistry of the Yubdo mafic-ultramafic rocks indicate that the body is an Alaskan-type intrusion.