Geochemical traits of Maastrichtian black shales with turbidites from Rogozna Mt. (SW Serbia): composition and provenance

The Cretaceous Period is notable for numerous occurences of organic-rich muds (Usman et al. 2025). These mudrocks are often termed „black shales“, due to the dark colour imparted by organic matter and sulphides, and often present fissility. Black shales of the Rogozna Mt. were deposited during the upper levels of the Maastrichtian Age (Urošević et al. 1973), representing the background pelagic sedimentation often intersected by turbiditic calcareous clastics, as well as debris flow breccias. They are thus considered the lower, mud-rich part of the rapidly advancing flysch complex (Anđelković 2026). These sediments were subsequently heavily deformed by the Alpine orogenesis.  

Rogozna is being explored by Zlatna Reka Resources (a subsidiary of Strickland Metals) for skarn-hosted and porphyry deposits, completing a large number of drillholes in the eastern parts of the mountain. Extensive laboratory assaying performed on all core proved a rich source of data for geochemical modelling. Several drillholes with high shale thickness were selected in order to illustrate properties of different prospects, and thus positions within the basin.

The calculated Index of Compositional Variability (after Cox et al, 1995) shows high values, signifying first-cycle deposition in a highly active tectonic regime. This is in accord with current understanding of the basin evolution (Schmid et al. 2020, Anđelković 2026). The values drop westward and with time, mostly the effect of the constant decrease in Ca content. It must be noted that within this lithofacial unit, Ca is mostly connected with turbiditic sandstones, but this distinction is masked by the fact that assay samples include both lithologies in bulk. K/Al ratios are higher in the older units as well, with Gradina North being especially enriched in K, and thus detrital feldspar. The mid-low ratios in younger levels indicate strong muscovite and illite mineralogy. Ti content in shales generally is considerably higher than in the underlying marly carbonates, and is stable within the unit. Lithogeochemical classification performed after the methodology of Ordóñez-Calderón et al. (2017) mostly puts the shales in the Siliciclastic-Crystalline category.

These results confirm that the transition from dominantly carbonate to dominantly sicliciclastic deposition was highly gradual, with the Ca decrease still ongoing after the change in lithofacies from marlstone to black shale with distal turbidites. Another implication is provenance: the sediment composition and depositional mode change coincides with the progressive erosion at the western fringe of the basin, where first Triassic carbonates, then Triassic mixed sediments, Permian clastics, and finally Paleozoic low-grade metamorphics were successively removed. This confirms the western basin edge as the main source area, and composition of the derived sediments is directly linked to the source material composition.