Concentrically zoned pyrite aggregates (CZPA) are common in sediment‐hosted massive sulfide(SHMS) deposits and have been widely used to interpret the ore‐forming processes. There is considerableuncertainty, however, over the formation of aggregates that are oscillatorily zoned and contain randomly‐orientated pyrite microcrystals. Guided by the results of examination of the micro‐textures of CZPA and in‐situchemical analyses, we conducted a quantitative diffusion‐reaction simulation to assess the mechanism of CZPAformation. Our study shows that oscillatory zoning results from the feedback between the diffusion of reactantsand the nucleation‐growth of Fe‐sulfides. Externally derived Fe2+ diffuses into the early diagenetic sedimentscontaining decomposing organic matter (2CH2O + SO42。