Newly found tuffic horizons in the Visean deposits of the Lublin Basin, SE Poland: petrological characteristics, origin and stratigraphic significance

Abstract

Detailed petrographical and mineralogical analyses are provided of tuffic horizons identified for the first time in Upper Visean deposits (sequences 2 and 3) in 21 boreholes of the Lublin Basin, along with co-occurring volcaniclastic rocks. The thickness of the tuff horizons varies in the range of 0.2–8 m. Tuff 2.I reaches its maximum thickness in the NE region, and tuffs 2.II and 3.I in the centre of the basin. As these tuffs can be traced between different parts of the basin, they provide good regional correlative horizons. Tuff horizons 2.I and 2.II have been identified within sequence 2, and tuff horizon 3.I in sequence 3. The rhyolitic pyroclastic material composing the tuffs was sourced from the eruptive acidic volcanism that took place in the Lublin Basin in the Late Visean. The pyroclastic material was deposited in various environments that co-occurred in the basin area at that time: fluvial floodplain, deltaic plain, shallow-water delta, shallow clay-dominated shelf, and carbonate shelf. Within the horizons studied, fine-ash vitric and vitric-crystal tuffs, and coarse-ash vitric and vitric-lithic tuffs have been identified. The main clay mineral in the tuffs is kaolinite, the content of which can exceed 80%. In some areas it is accompanied by serpentine – berthierine, whereas in others, mixed-layered illite/smectite minerals and illite predominate. Anatase and carbonate minerals of siderite, sideroplesite and Fe/Mn-calcite are common. Hematite, pyrite and jarosite are also sometimes found. The recent composition of the tuffs results mainly from the type of pyroclastic material, as well as from the factors it has been subjected to, including sedimentary environment, hypergenic weathering, and hydrothermal and diagenetic processes. The detrital components found in the volcaniclastic conglomerates and sandstones were derived mostly from erosion and reworking of rocks from sequences 1 and 2 occurring within the sedimentary basin, and to a lesser extent they came from outside of the basin. Preliminary mineral resource studies have shown that the tuffic horizons can contain ~28–35% of Al2O3, which is a prospective quantity for refractory raw materials. The volcaniclastic rocks, in which enrichment in rare earth elements is found, also show the potential for being economic raw materials. The conclusions about the raw material potential should be considered preliminary, requiring further research.