Biomarkers for Tracking Organic Matter Maturity in Therapeutic Muds (Peloids): A Comparison of Natural and Spa-Scaled Systems

Peloid maturation is governed by geological settings, duration of water–sediment interaction (microbial reworking), and thermomineral water properties, with molecular distributions providing insights into transformation mechanisms. To assess site-specific biomarker maturity, geochemical parameters were applied to five Serbian therapeutic muds, including spa peloids (Bujanovac, Jošanica, Kanjiža) and natural systems (Vrujci, Rusanda). Mineralogy was determined via X-ray diffraction, and organic matter (OM) was characterized by gas chromatography–mass spectrometry of n-alkanes, steranes and hopanes. Samples are mostly clay-rich, providing favorable catalytic conditions for biomarker transformations. Bujanovac shows a higher plant OM signature (n-C29 maximum) and elevated biomarker maturity (high Ts/Tm, near-equilibrium C29 S/(S + R)), likely inherited from volcanically influenced source material. Jošanica exhibits high CPI but low Ts/Tm and C29 S/(S + R), indicating largely immature OM despite four years of spa aging. Kanjiža shows unexpectedly high apparent maturity after one-day aging, with a pronounced UCM and C31 S/(S + R) (0.58), consistent with incorporation of migrated petrogenic hydrocarbons. Vrujci displays coherent maturity due to prolonged water–sediment interaction, clay-rich mineralogy, extended aging, and regional geothermal gradients. Rusanda exhibits decoupled parameters (CPI 3.91, C29 S/(S + R) 0.69), indicative of hydrocarbon overprinting. Overall, integrating biomarker geochemistry with mineralogy, depositional context, and local thermal/geological conditions provides a robust framework to evaluate peloid maturation.