Massive hematite-Mn ores of the Gonzen mountain near Sargans, Switzerland show various features indicating a synsedimentary origin. These include vertical and lateral transition into pelagic limestones, layers and laminae ranging from 1 to 20 mm in thickness, and embedded microfossils. Bulk rock composition and stable isotope analyses of carbonates point to a hydrothermal origin comparable to the situation in modern marine environments with high heat flow and submarine volcanism: These analyses show (1) low contents of Al, Ni, Cu, Co, U, Th and Ce and high values of Fe or Mn, SiO 2 and Ba; and (2) light carbon isotope values in carbonate-bearing ores (−2 to −5‰) which contain only little or no organic carbon. However, the paleogeographic location of the Helvetic realm (passive continental margin) and crustal lead isotope ratios exclude a volcanic origin but suggest that the mineralising fluid has its origin in the underlying continental basement. The existence of these peculiar ore deposits seems to indicate a Late Jurassic reactivation of a local fault system connected with the emplacement of ophiolites in the nearby North Pennine Basin, a branch of the Tethyan Ocean. However, the only signs of faulting in the area are thin limestone breccia beds, occurring below and above the ore bodies.