DIAGENESIS OF DIATOMITE FROM THE KOLUBARA COAL BASIN, BAROSEVAC, SERBIA

Diatomite associated with the Kolubara Coal Basin was studied to bette r understand early stage silica diagenesis of shallow water deposits. The Kolubara Basin consists of Neogene siliciclastic rocks, diatomite, marlstone and rare carbonates. Palaeozoic metamorphic and Mesozoic se dimentary and igneous basement rocks are transgressively overlain by U pper Miocene sandstone, siltstone, shale and mudstone. This Upper Mioc ene section is transgressively overlain by the Pontian section, which contains diatomite and coal beds. White and grey diatomite forms beds 0.7-2.2 m thick that are continuous over an area of about 2 km2. Silic eous rocks vary in composition from diatomite (81-89 per cent SiO2) to diatom-bearing shale (58-60 per cent SiO2). Siliceous deposits are la minated in places, with the laminae defined by variations in clay mine rals, organic matter and diatoms. Diatomite shows only incipient diage nesis characterized by the fragmentation of diatom frustules, the mino r to moderate corrosion of frustules and the formation of minor amount s of opal-A' (X-ray amorphous inorganic opal) cement. The low degree o f diagenesis results from the young age of the deposits, low burial te mperatures and possibly also from the presence of abundant organic mat ter and the dissolution of kaolinite. The presence of only weak diagen esis is also reflected by the characteristically poor consolidation of the rocks and low rank of the associated coal.