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 considerable uncertainty, 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‐situ chemical analyses, we conducted a quantitative diffusion‐reaction simulation to assess the mechanism of CZPA formation. Our study shows that oscillatory zoning results from the feedback between the diffusion of reactants and the nucleation‐growth of Fe‐sulfides. Externally derived Fe2+ diffuses into the early diagenetic sediments containing decomposing organic matter (2CH2O + SO42− = 2HCO3− + H2S) and reacts with H2S to form a pyrite layer via an intermediate pathway (Fe2+ + H2S → FeS + 2H+, FeS + H2S → FeS2 + H2). This growth of pyrite layers depletes the local concentration of reactants and suppresses nucleation until the diffusive reaction front advances and another layer is formed. Intermediate phases, for example, mackinawite, nucleate instead of pyrite, because of their greater ease of nucleation due to the low surface tension, and lead to the domination of nucleation over growth. The nucleation of mackinawite and occurrence of siderite in the CZPA are consistent with a low temperature, high pH, anoxic early diagenetic environment. Our study demonstrates that CZPA in SHMS deposits are formed by intrinsic self‐organizational processes rather than by extrinsic changes of ore‐forming fluids. The CZPA in SHMS deposits are thus indicative of their diagenetic origin with Fe2+ infiltrated and diffused from hydrothermal fluids into the sediments. Plain Language Summary: Oscillatory zoning patterns are a phenomenon commonly encountered in scientific disciplines that include biology, geology, chemistry and atmospheric physics. To decipher these patterns, we employ a unifying principle that leverages a reaction‐diffusion approach and successfully replicate the textures observed in a massive sulfide deposit, via numerical simulations. In addition, we show that factors such as the concentration, distribution, and mobility of reactant species, such as the iron ion, sulfide, and organic matter in sediments, influence the texture of patterns at both the millimeter and nanometer scales. Our proposed numerical models yield results that closely align with the oscillatory zoning patterns observed in pyrite‐siderite from the Xinqiao massive sulfide deposit in Central China. The broader impact of this work lies in the development of a computer simulation‐based protocol, which sheds light on the processes forming massive sulfide deposits. This, in turn, has the potential to significantly aid the exploration for new metal and sulfide resources beneath the surface of the Earth. Key Points: Reactive‐diffusion simulations of the formation of concentrically zoned pyrite aggregates in sediment‐hosted massive sulfide depositsThe ore textures were the result of an intrinsic self‐organized process rather than extrinsic changes of chemical compositionsProtocols for deciphering diagenetic environments affected by the infiltration of Fe2+‐rich hydrothermal fluids [ABSTRACT FROM AUTHOR]