Origin Of opal Hosted in Volcanic Rocks At Koste – Amba and Gift Mariyam, North Shoa (Central – Eastern Ethiopia)
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Date
2006-07
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Addis Ababa Universty
Abstract
Gemstone- quality opal in North Shoa /koste - Amba & Gift – Maryam localities is found
in small amount. The opal – bearing rhyolitic glass is found situated at the topmost part
of the Ethiopian flood basalt. It is sandwiched between an underlying rhyolite and an
overlying volcanic lava composed of some stacks of superimposed basalt flows at the
immediate top / just next to the weathered mantle of the rhyolitic glass.
The main objective of the present study is to examine the origin of volcanic – hosted
opal in Koste – Amba & Gift – Mariyam /North – Shoa, Amara - Region using
petrographic description, major & trace element analysis, as well as x –ray diffraction(for
samples collected from surficially altered part of the opal bearing rhyolitic glass/volcanic
glass). Besides, it has been intended to establish the geological and geochemical
conditions responsible for opal mineralization.
The deposit is associated with surfically altered rhyolitic glass, i.e. this lithologic unit has
the same mineralogical composition with the rhyolite. A cliff forming rhyolite
characterized by alternating vesicular subunit with finely laminated subunit on the same
lithology underlies the opal – bearing rhyolitic glass. The size of the vesicles in this
underlying rhyolite increases from bottom to top. However, these vesicles are not filled
with any secondary minerals like opal.
Generally, opal accommodating lithologic unit of the study area is the rhyolitic glass/
volcanic glass, so that the mineralization seems to be highly influenced by lithology and
other physico-chemical factors.
Features related to opal, like gemmology, genesis (formation environments, associated
minerals and rocks) are elaborated based on cumulative global findings
Using results of major element analysis and TAS classification diagram, fresh rock
samples 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 that
presently accommodate opal.
Samples have been taken from mantle material as well as from the fresh rhyolitic glass.
According to the petrographic description, both fresh and mantle samples of the rhyolitic
glass are found to have Sanidine and pyroxene main primary rock forming silicate
minerals as well as zeolite. Pyroxene is found in trace amounts in the mantle material of
the rhyolitIc glass but it is about 4% in the fresh rhyolitic glass. Besides both fresh &
mantled rhyolitic glass contains high amount of volcanic glass and lesser amounts of
opaque minerals & rock fragments.
X-ray diffraction analysis that has been done on samples from the highly altered top part
of the rhyolitic glass shows clay minerals (montmorillonite and kaolinite/ halloysite),
zeolite minerals (Philippsite & mordenite), as well as silica (quartz & cristobalite).
Deep surface weathering and a lack of detailed site investigations may obscure the
difference between surface weathering process and post – volcanic hydrothermal
activity. In the presence of this fact, (a) the formation of montmorillonite possibly from
simultaneous surface weathering of sanidine and pyroxene that are contained in the
rhyolitic glass or from the devitrification of volcanic glass fragments(shards)
accompanied by the release of surplus silica to surrounding waters for the creation of
opal mineralization in the study area ; (b) the occurrence of zeolite minerals(mordenite
& philippsite) usually in earth surface weathering environments(with some exceptions
where philippsite can occurs in saline - lake deposits, calcareous deep – sea sediments,
hot – spring deposits) characterized by percolating ground water in acidic tephra; (c) the
existence of opal – empted vesicular rhyolite and successive lava flows that underlies
and overlies respectively the opal bearing horizon; all these together may exhibit that
silica – rich waters derived from surface weathering processes can be the origin of opal
in Koste – Amba & Gift – Mariyam localities.
Besides, recommendation as what is to be done regarding further research works on the
deposit is forwarded.
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Keywords
North Shoa /koste - Amba & Gift – Maryam