[en] Roman marble quarry in Bistrica gorge in southern Pohorje Mt. (north-eastern Slovenia) is situated in a 20 m thick lens of layered marble, at the contact zone between granodiorite and metamorphites. Grey and yellowish non-homogenous calcite marble is heavily included by mica, quartz, feldspars, zoisite, pyrite and amphiboles. In the present research, we have studied numerous pyrite (FeS_2) crystals associated with yellowish-bronze non-stoichiometric pyrrhotite (Fe_1_−_xS), not previously reported from this locality. SEM investigation revealed unusual sequence of crystallisation: primary skeletal pyrrhotite matrix is sparsely overgrown by well-crystalline pyrite, both being overgrown by smaller, well-developed hexagonal pyrrhotite crystals of the second generation. With TEM we identify the pyrrhotite as 5T-Fe_1_-_xS phase, where x is about 0.1 and is equivalent to Fe_9S_1_0. The pyrite-pyrrhotite coexistence allows us a construction of fO_2-pH diagram of stability fields, which reflects geochemical conditions at the time of marble re-crystallisation. (paper)