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Geochemical and petrological characterization of gem opals from Wegel Tena, Wollo, Ethiopia: opal formation in an Oligocene soil
<p>Gem opals from Wegel Tena, Wollo Province, Ethiopia, occur in Oligocene rhyolitic ignimbrites. They display a unique geochemistry,
with some samples yielding the highest Ba concentrations ever recorded. They are generally much richer in chemical impurities
than opals from other localities. For example, the sum Al+Fe or the sum Na+Mg+Ca+K+Ba are often higher. These geochemical
features make them easy to distinguish from other opals worldwide. We observed strong geochemical variations and some good
positive correlations in our samples, such as Al+Fe vs. Na+Mg+Ca+K+Ba, Al vs Ca, or Ba vs Ca. This shows that the crystallography
of opal has controlled, at least in part, the incorporation of chemical impurities, although opal is not well-crystallized.
In addition, the multimodal distributions of several chemical impurities (e.g. U vs Sr, Al vs Ca, Ba vs Ca, etc.) suggest
at least two origins of silica: weathering of feldspars and weathering of volcanic glass. In addition, opals from Wegel Tena
contain numerous well-preserved microscopic plant fossils. Moreover, their host rock exhibits features typical of pedogenesis
(abundant clays, desiccation cracks, and grain size sorting). We propose that the opals at Wegel Tena formed during the Oligocene
period when volcanic emissions stopped for a time long enough to allow weathering of ingimbrites and therefore liberation
of silica. This accompanied the formation of soil and development of plant life, and some plants were trapped in opal.
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