Trace and heavy element distribution of the Hwange Coals in Zimbabwe
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Date
2013
Journal Title
Journal ISSN
Volume Title
Publisher
University of Namibia
Abstract
The study of heavy metals in the Hwange coals enhances knowledge of both the geological evolution
of the coal-bearing horizons and the climates that prevailed during their deposition, as
well as indicating likely trace and heavy elements that would affect the health of the miners
and the environment at large. The Main Coal Seam of the Hwange coals are bracketed by the
Lower Wankie Sandstones of Carboniferous to Permian that were deposited during the Dwyka
epoch and the Upper Wankie Sandstones of Permian age. The Hwange coals of Zimbabwe are
inferred to have been deposited in swamps associated with fresh water lakes lying on either side
of a great elevated tract of Archean and Proterozoic basement during the early Permian. Initially
the lake was shallow, as observed from sedimentological data, and eventually the lake became
deeper during the continuation of the Karoo period of southern Africa. The sedimentation in
Zimbabwe began with the melting of the Carboniferous ice cap of the then south and central
Africa. The Karoo succession in the Mid-Zambezi Valley suggests that climatic cycles of glacial
to semi-glacial and finally to post-glacial nature occurred, followed by very hot, humid and arid
conditions towards the end of the sedimentation period. Four sedimentary successions are reflected
at Hwange, the first being fluvial-glacial beds, followed by flooding and deposition of
the Lower Wankie Sandstones. At the end of this phase, there followed a general increase in
flora and fauna, responsible for the formation of coal now preserved in the Main Coal Seam.
Above this succession is the Lower Carbonaceous Mudstones of the Wankie succession. The
unconformity of the Upper Wankie Sandstones with the Lower Mudstones marks the end of the
coal-bearing horizons. Above the Upper Wankie Sandstones further flooding at a large scale occurred,
which is thought to have been rapid, as no coal seams are associated with this mudstone
in the whole of the Zambezi valley. A systematic study of heavy elements, ash, moisture content
and arsenic, shows that during the deposition of the Hwange coals of the Main Coal Seam
there were significant amounts of gallium, germanium, niobium, vanadium and chromium in the source areas, together with metals of felsic affinity such as strontium, tin and lithium. There is
a systematic relationship between ash content and heavy mineral content in the coal. For coals
with a high heavy metal content, their ash contents are low in some samples, whereas others
demonstrate the reverse. While we can easily attribute the source of the felsic affliated elements
to the basement granites, heavy metals are interpreted to have been fixed under anoxic conditions
when the organic matter was being converted to coal. It is suggested that the heavy elements may
have acted as catalysts in the coalification processes.
Description
Keywords
Karoo, Heavy elements, Ash content, Glaciations, Climate, Gallium, Germanium, Lithophile, Chalcophile, Siderophile
Citation
Mapani, B. and others. 2013. Trace and heavy element distribution of the Hwange Coals in Zimbabwe. International Science and Technology Journal of Namibia 1(1-2):89-105.