Opening-mode veins in cores drilled from the mudrocks overlying and underlying the major Silurian salt décollement in the Appalachian plateau (Tioga and Lawrence Counties, Pennsylvania) have mineralogic and isotopic compositions generally matching those of their host mudrocks, suggesting opening and filling amid little cross-stratal fluid motion. Calcite and most trace minerals probably entered the veins via dissolution–reprecipitation from nearby host rock. Consistent with this interpretation are the observations that (1) trace minerals within the veins, including quartz, pyrite, and dolomite, are invariably also present within the layers hosting the veins, with vein cement minerals generally reflecting the abundance and solubility of minerals in the host rock, and (2) carbon and oxygen isotopic compositions of vein-filling calcite are similar to those of calcite within the host rock, with vein-filling δ18O slightly depleted and δ13C slightly enriched. Modeling the fluid isotopic evolution, assuming vein opening and filling amid immobile connate formation water, accounts for these minor but systematic differences, which are attributable to increasing temperature and hydrocarbon maturation. An exception to the above trend is barite, which, despite its low solubility, is systematically enriched in veins with respect to the host rock. It is unclear whether barite precipitation resulted from the influx of external fluids—perhaps deriving from Silurian salt—or from barium mobilized at depth from local clays or organic material.