Abstract The Reocin deposit, which displays no visible relationship with a magmatic source, is located in the upper, dolomite-rich part of the Aptian series and more precisely in the lower 40 m of the dolomite-rich rocks. Sedimentology and mineralography, as well as other approaches are used to define the environment of deposition. In short, the following generalizations hold: the only mineralized rocks are the Upper Aptian dolostones; the mineralization is stratigraphically controlled; where the dolostones become thinner and pinch out, so do the ore beds. But not all dolostones are mineralized. The three ore beds found in the Central Area thin westward and more markedly eastward. The mineralogy, with sphalerite and marcasite as major constituents, galena, melnicovite-pyrite and pyrite as minor constituents, remains identical in the three ore beds and the three mining areas. In the Western Area, shallower facies prevail with breccias and breccia-like marls, respectively, around and inside a barren region. In the Eastern Area. the emplacement of the ore is ruled by deeper facies. In addition to large scale sedimentological controls, the presence of thin marly beds with sulfides, as well as micro-textures and micro-structures, suggest syngenetic deposition. Moreover, the ore-beds display deformations which obviously occurred in an unconsolidated material. Cobalt to nickel ratios in marcasite, pyrite and melnicovite, as well as the absence of thallium in these sulfides, are in favor of a sedimentary origin. Contrary to earlier work, the present results support a syngenetic sedimentary hypothesis.