Origin Of Opal Hosted In Volcanic Rocks At Koste Amba And Gift Mariyam North Shoa (central Eastern Ethiopia)

Gemstone- quality opal in North Shoa /koste - Amba & Gift – Maryam localities is foundrnin small amount. The opal – bearing rhyolitic glass is found situated at the topmost partrnof the Ethiopian flood basalt. It is sandwiched between an underlying rhyolite and anrnoverlying volcanic lava composed of some stacks of superimposed basalt flows at thernimmediate top / just next to the weathered mantle of the rhyolitic glass.rnThe main objective of the present study is to examine the origin of volcanic – hostedrnopal in Koste – Amba & Gift – Mariyam /North – Shoa, Amara - Region usingrnpetrographic description, major & trace element analysis, as well as x –ray diffraction(forrnsamples collected from surficially altered part of the opal bearing rhyolitic glass/volcanicrnglass). Besides, it has been intended to establish the geological and geochemicalrnconditions responsible for opal mineralization.rnThe deposit is associated with surfically altered rhyolitic glass, i.e. this lithologic unit hasrnthe same mineralogical composition with the rhyolite. A cliff forming rhyoliterncharacterized by alternating vesicular subunit with finely laminated subunit on the samernlithology underlies the opal – bearing rhyolitic glass. The size of the vesicles in thisrnunderlying rhyolite increases from bottom to top. However, these vesicles are not filledrnwith any secondary minerals like opal.rnGenerally, opal accommodating lithologic unit of the study area is the rhyolitic glass/rnvolcanic glass, so that the mineralization seems to be highly influenced by lithology andrnother physico-chemical factors.rnFeatures related to opal, like gemmology, genesis (formation environments, associatedrnminerals and rocks) are elaborated based on cumulative global findingsrnUsing results of major element analysis and TAS classification diagram, fresh rockrnsamples are plotted in the field of basalt and rhyolite. Therefore, the host & parent rock of amorphous silica (SiO2 . nH2O) seems to be the rhyolitic glass /volcanic glass thatrnpresently accommodate opal.rnSamples have been taken from mantle material as well as from the fresh rhyolitic glass.rnAccording to the petrographic description, both fresh and mantle samples of the rhyoliticrnglass are found to have Sanidine and pyroxene main primary rock forming silicaternminerals as well as zeolite. Pyroxene is found in trace amounts in the mantle material ofrnthe rhyolitIc glass but it is about 4% in the fresh rhyolitic glass. Besides both fresh &rnmantled rhyolitic glass contains high amount of volcanic glass and lesser amounts ofrnopaque minerals & rock fragments.rnX-ray diffraction analysis that has been done on samples from the highly altered top partrnof the rhyolitic glass shows clay minerals (montmorillonite and kaolinite/ halloysite),rnzeolite minerals (Philippsite & mordenite), as well as silica (quartz & cristobalite).rnDeep surface weathering and a lack of detailed site investigations may obscure therndifference between surface weathering process and post – volcanic hydrothermalrnactivity. In the presence of this fact, (a) the formation of montmorillonite possibly fromrnsimultaneous surface weathering of sanidine and pyroxene that are contained in thernrhyolitic glass or from the devitrification of volcanic glass fragments(shards)rnaccompanied by the release of surplus silica to surrounding waters for the creation ofrnopal mineralization in the study area ; (b) the occurrence of zeolite minerals(mordenitern& philippsite) usually in earth surface weathering environments(with some exceptionsrnwhere philippsite can occurs in saline - lake deposits, calcareous deep – sea sediments,rnhot – spring deposits) characterized by percolating ground water in acidic tephra; (c) thernexistence of opal – empted vesicular rhyolite and successive lava flows that underliesrnand overlies respectively the opal bearing horizon; all these together may exhibit thatrnsilica – rich waters derived from surface weathering processes can be the origin of opalrnin Koste – Amba & Gift – Mariyam localities.rnBesides, recommendation as what is to be done regarding further research works on therndeposit is forwarded.

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