Trace and heavy element distribution of the Hwange Coals in Zimbabwe

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.