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.

中文翻译：

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沉积物块状硫化物矿床中菱铁矿和黄铁矿同心带聚集体自组织成因的数值模拟

同心带状黄铁矿聚集体（CZPA）在沉积物块状硫化物（SHMS）矿床中很常见，并已被广泛用于解释成矿过程。然而，对于振荡分区并含有随机取向的黄铁矿微晶的聚集体的形成存在相当大的不确定性。在 CZPA 微观结构检查和原位化学分析结果的指导下，我们进行了定量扩散反应模拟，以评估 CZPA 形成的机制。我们的研究表明，振荡分区是反应物扩散与铁硫化物成核生长之间反馈的结果。外来Fe ²⁺扩散到含有分解有机质（2CH ₂ O + SO ₄ ²⁻  = 2HCO ₃ ⁻  + H ₂ S）的早期成岩沉积物中，并通过中间途径（Fe）与H ₂ S反应形成黄铁矿层。²⁺  + H ₂ S → FeS + 2H ⁺，FeS + H ₂ S → FeS ₂  + H ₂ )。黄铁矿层的生长耗尽了反应物的局部浓度并抑制成核，直到扩散反应前沿前进并形成另一层。中间相，例如麦基纳铁矿，代替黄铁矿成核，因为它们的表面张力低，更容易成核，并导致成核对生长的主导。CZPA 中镁铝矿的成核和菱铁矿的赋存与低温、高 pH、缺氧的早期成岩环境相一致。我们的研究表明，SHMS 矿床中的 CZPA 是由内在的自组织过程形成的，而不是由成矿流体的外在变化形成的。因此，SHMS 沉积物中的 CZPA 表明其成岩起源是 Fe ²⁺从热液渗透并扩散到沉积物中。