Terrigenous organic matter and formation of siderite in the Bathonian Ore-Bearing Clay Formation at Gnaszyn, Poland – a petrochemical study

Abstract

The Bathonian Ore-Bearing Clay Formation, outcropping in the Gnaszyn open-pit mine at Częstochowa (Poland), includes several horizons of abundant iron carbonate concretions . The cores of the concretionary bodies commonly contain organic matter (OM), dominated by fragments of wood. These organic particles usually display well-preserved primary structures and occur rarely as more deformed and/or completely degraded fragments. Their original structures are frequently replaced by or filled with secondary mineralization, mostly represented by pyrite. The maceral composition of the OM of the wood fragments is dominated by huminite with subordinate inertinite and resinite. Vitrinite reflectance analyses revealed values lower than 0.45%. The total organic carbon content (TOC) displayed variable results between 2% and 18%. Rock-Eval analyses revealed low amounts of hydrogen (< 45 mg HC/g TOC) and relatively high amounts of oxygen (up to 136 mg CO 2 /g TOC). Analysed samples contained small quantities of free hydrocarbons (S1 peak < 0.26 mg HC/g rock) as well as hydrocarbons, generated during pyrolysis (S2 peak < 7.05 mg HC/g rock). These features are characteristic for immature type IV kerogen of terrigenous origin. However, the maceral composition and frequent occurrence of siderite affecting the Rock-Eval parameters may indicate that the original kerogen belonged to type III. According to previous authors, the OM of terrigenous origin was delivered to well-oxygenated water of the palaeo-basin in the Częstochowa area. The present data indicate that intensive biodegradation of this OM at shallow burial depleted the oxygen supply within the sediment, driving the pore water into dys- or anoxic conditions. The activity of microorganisms in reducing iron and/or sulphates became the dominant biodegradation reaction, introducing Fe 2+ and HCO 3 - ions into the system. Negative δ 13 C values in the cortex of the concretions analysed indicate that the bicarbonate consumed in siderite precipitation was supplied by this microbial activity. The reducing microenvironments developed in the sediment and wood fragments acted as nucleation sites for siderite precipitation.