The geology and geochemistry of the Madzaringwe formation in the Vele colliery, Tuli coalfied, Limpopo Province, South Africa

Abstract

The Madzaringwe Formation in the Tuli Basin, Limpopo Province of South Africa is one of the coal-bearing late Palaeozoic units of the Karoo Supergroup. The Madzaringwe Formation in the former Vele colliery (now referred to as Vele MC Mining) is the focus of this study and it consists of sandstone and shale with thin coal seams. To date, published data gives an overview of the stratigraphy of the Madzaringwe Formation in the Tuli coalfield. Little is known of the petrographic characteristics, lithofacies and geochemistry of the Madzaringwe Formation. Hence, this research work is undertaken to better define the lithological characteristics, provenance and tectonic setting of the Madzaringwe Formation in the Vele Colliery, Limpopo Province, South Africa. In the Vele colliery, the Tshidzi Formation forms the base of the Karoo sequence, consisting of diamictite and attains a maximum thickness of about 10 m. The Madzaringwe Formation overlies the Tshidzi Formation and it is made up of shale, mudstone and sandstones with subordinate siltstones and coal seams. Succeeding the Madzaringwe Formation is the Mikambeni Formation, comprising of black carbonaceous shale, mudstone and sandstones with minor coal layers. The Fripp Formation overlies the Mikambeni Formation and it consists of sandstones with subordinate grey mudstones and attains a maximum thickness of about 15 m. The Madzaringwe, Mikambani and Fripp Formations in the Vele colliery can be correlated with the Ecca Group of the main Karoo Basin. Based on the detailed sedimentological analyses of borehole and open pit data, ten lithofacies were identified and four facies associations (FAs) were recognised. The facies associations are: FA 1: Carbonaceous and pyritic shale and mudstones (Fls + Fss), FA 2: Black coal and shaly coal (C + Cs), FA 3: Dark grey micaceous and calcareous shale and mudstone, with subordinate siltstones (Fls + Fm, Fc + Fms + Fmb), and FA 4: Siltstone intercalated with fine to coarse grained sandstones (Fms + Fss + Sm + D). Sedimentological characteristics of the identified facies associations indicate shallow lake and floodplain depositional environments. The petrographic characterization revealed that vitrinite is the dominant maceral group in the coals, making up to 81-92 vol.% (mmf) of the total sample. Collotellinite is the dominant vitrinite maceral, with total count varying between 52.4 vol.% (mmf) and 74.9 vol.% (mmf), followed by corpogelinite, collodetrinite, tellinite and pseudovitrinite with count ranging between 0.8-19.4 vol.% (mmf), 1.5-17.5 vol.% (mmf), 0.8-6.5 vol.% (mmf) and 0.3-5.9 vol.% (mmf), respectively. The dominance of collotellinite gives a clear indication that the coals are derived from the parenchymatous and woody tissues of roots, stems and leaves. The mean random vitrinite reflectance values range between 0.75 and 0.76%, placing the coals in the medium rank category (also known as the high volatile bituminous coal) based on the UN-ECE coal classification scheme. The high amount of inertinite, especially fusinite with empty cells and semifusinite in the coals will pose a threat to coal mining because it aids the formation of dust. The sandstones of the Madzaringwe Formation are classified as sub-arkoses and sub-litharenites. Petrographic and geochemical analyses of the mudrocks and sandstones show that the rocks are from silicic or felsic igneous rocks. The tectonic setting discrimination diagrams support passive-active continental margin settings of the provenance. The indices of weathering/alteration and the binary plot of the index of compositional variability (ICV) against chemical index of alteration shows that the studied samples have been subjected to moderate to intensive weathering. Keywords: Geology, geochemistry, Madzaringwe Formation, Tuli Coalfield, South Africa