GEOCHEMICAL INSIGHT ON GEM OPAL FORMATION IH HIGHLY WEATHERED RHYOLITIC IGNIMBRITE LAYER FROM DELANTA AREA, SOUTH WOLLO, NORTHERN ETHIOPIA

A gem-quality opal is found in Delanta Woreda especially in Wegel Tena, and Tsehay Mewucha locality. The region consists of a thick (>3,000 m) volcanic sequence of basalt and rhyolitic ignimbrite. Over this volcanic series, only one very thin layer (<3 m thick), of rhyolitic ignimbrite is mineralized with opal. The main objectives of this study were understanding the relationship between opal with its host rock and propose genetic model for opal. It is examined using petrographic description, and whole-rock geochemistry (major and trace element analysis) of the host rhyolitic ignimbrite and opal. To achieve this, samples are directly extracted from three different local mining sites of Delanta area (Tantakoa to the northeast, Worke Washa to the east, and Berbere Wenz to the south of the study are) from the opal bearing layer. Petrographically, the matrix is composed of glassy, non-crystalline, and amorphous groundmass (>75%), occasionally welded and mostly weathered with varying size phenocrysts of quartz, plagioclase, and sanidine. Regarding the result of trace element geochemistry, rhyolitic ignimbrite and opal samples show variable characteristics always a lesser concentration in opal. The presence and concentration of trace elements in opals reflect primarily the host-rock composition, as silica in opal comes from its weathering