Platinum and Gold Deposit of Yubdo Mafic-Ultramafics, Western Ethiopia: A Late Stage Hydrothermal Mineralization

ABSTRACT
An estimated 2700 kg of platinum (Pt) has been extracted from
the Yubdo platinum deposit during its mining history and it still has
an estimated reserve of 12 tons of crude Pt with an average Pt content
of 0.336 g/t in the lateritic cover. Recent data indicate that the
weathered bedrock of serpentinized dunite contains an overall average
of 0.59 g/t Pt and 0.23 g/t Au.
A total of 19 platinum-group mineral (PGM) inclusions are
identified in the Pt-Fe nuggets of Alfe and Birbir rivers, which drain
the Yubdo belt. Except for the Os-Ir and tulameenite inclusions, all
other PGM inclusions are associated with sulfides and low-temperature
hydrothermal minerals of arsenides, antimonides, sulfoarsenides,
sulfoantimonides and bismuthotellurides. The analyses
of two borehole sections for PGE and gold show that the gold values
increase with the increment of platinum and palladium in the
serpentinized zone. The highest values of platinum-group elements
(PGE) and gold are related to the section where PGM are identified
by microscopic and electron microprobe studies.
Most of the placer gold nuggets from Alfe and Birbir rivers
have a composition of 80 to 85 at. % Au and 15 to 20 at. % Ag, while
a few nuggets have electrum composition with up to 52 at. % Ag and
46 at. % Au. The shape of the gold and Pt-Fe nuggets of Yubdo have
typical elongated and leaf-like shapes characteristic of hydrothermal
type mineralization.
The different chemistry of the PGM inclusions and their association
with secondary sulfides; the discovery of the primary PGM
in a serpentinized section of the dunite, and the shape of Pt-Fe and
Au nuggets suggest that the PGM and Au are remobilized from the
mafic-ultramafic rocks by a late stage hydrothermal fluid and possibly
related to a diorite intrusion that cuts through the mafic-ultramafic
rocks of Yubdo.