The Precious Opal Deposit At Wegel Tena, Ethiopia: Formation VIA Successive Pedogenesis Events

Research areas:

• Planète Terre

Year:

2017

Authors:

• Boris Chauviré
• Benjamin Rondeau
• Francesco Mazzero
• Dereje Ayalew

Journal:

The Canadian Mineralogist

Volume:

55

Number:

4

Pages:

701

BibTex:

@article{doi:10.3749/canmin.1700010, author = "Boris Chauvir{\'e} and Benjamin Rondeau and Francesco Mazzero and Dereje Ayalew", abstract = "Since its discovery in 2008, the opal deposit in the Wegel Tena area of Ethiopia, located within an Oligocene ignimbritic trap, has become a major provider of precious opal. This study provides new field evidence and laboratory results that reinforce the pedogenetic model for gem opal formation. In the eight mines investigated, opal occurs in lenticular horizons within an unwelded ignimbrite bed. Moreover, several deposits are superimposed in the sequence, indicating that gem opal formation occurred several times in the same place. We observed that, in contrast to the other beds, the opal-bearing horizons are strongly weathered: volcanic glass is altered to smectite, rocks with a granular microstructure are apparent, and cavities are covered by illuvial clays. These features are typical of pedogenetic processes. Plant fossils in the opals reveal a well preserved cellular structure composed of chalcedony. These observations are consistent over all the mines, supporting the pedogenetic model for precious opal formation in the Wegel Tena area.", doi = "10.3749/canmin.1700010", extra = "PICL", journal = "The Canadian Mineralogist", number = "4", pages = "701", title = "{T}he {P}recious {O}pal {D}eposit {A}t {W}egel {T}ena, {E}thiopia: {F}ormation {VIA} {S}uccessive {P}edogenesis {E}vents", url = "http://dx.doi.org/10.3749/canmin.1700010", volume = "55", year = "2017", }

Abstract:

Since its discovery in 2008, the opal deposit in the Wegel Tena area of Ethiopia, located within an Oligocene ignimbritic
trap, has become a major provider of precious opal. This study provides new field evidence and laboratory results that reinforce
the pedogenetic model for gem opal formation. In the eight mines investigated, opal occurs in lenticular horizons within an
unwelded ignimbrite bed. Moreover, several deposits are superimposed in the sequence, indicating that gem opal formation
occurred several times in the same place. We observed that, in contrast to the other beds, the opal-bearing horizons are strongly
weathered: volcanic glass is altered to smectite, rocks with a granular microstructure are apparent, and cavities are covered by
illuvial clays. These features are typical of pedogenetic processes. Plant fossils in the opals reveal a well preserved cellular
structure composed of chalcedony. These observations are consistent over all the mines, supporting the pedogenetic model for
precious opal formation in the Wegel Tena area.