Geochemistry of diagenetic pyrite concretions from Late Oligocene marlstones at Črnova near Velenje, NE Slovenia: Evidence for sulfate-limited diagenetic conditions

Pyrite concretions within Late Oligocene marlstones from Črnova record a transition from open- to closed-system diagenetic conditions during burial. Two distinct zones are recognized: a syngenetic to early diagenetic cementation core and a later displacive rim, distinguished by texture, trace element composition, and sulfur isotope signatures. The core exhibits elevated trace element concentrations (e.g., V, Mn, Co, Ni, Cu, As, Mo) and δ 34 S values close to contemporaneous seawater sulfate, consistent with bacterially mediated sulfate reduction under conditions of active porewater exchange. In contrast, the rim shows markedly lower trace element concentrations and very heavy δ 34 S values, indicating severe sulfate limitation under strongly diffusion-restricted, near-closed conditions during late-stage growth. Together, these data record a shift from an initially sulfate-replete, open system with continuous porewater exchange to a diffusion-limited, near-closed regime in which sulfate replenishment could not keep pace with microbial sulfate reduction. The Črnova pyrite concretions demonstrate how pyrite geochemistry captures the transition from early diagenetic conditions to burial-induced system closure, providing a robust proxy for diagenetic evolution.