Compositional zoning is common in magnetite from different geological environments, but its formation mechanism remains controversial. Here, we characterize micron- to nano-scale textural and chemical variations in zoned magnetite from the Sossego iron oxide-copper–gold deposit (Carajás, Brazil) using electron probe microanalyzer (EPMA) and transmission electron microscopy (TEM). Lack of porosity and of a reaction front at both the micron and nanometer scales indicates that compositional zoning in magnetite is a pristine texture formed during crystal growth, rather than a secondary texture due to dissolution and reprecipitation reaction. TEM energy dispersive X-ray spectrometry analyses and mapping identify four types of nanoinclusions in zoned magnetite: 1) Mg-Fe-Al silicates, most likely amphibole, 2) Fe-Ti oxides, mainly ilmenite, 3) pyroxene, 4) Si-rich magnetite. The formation of ilmenite nanoinclusions in Sossego magnetite is possibly due to oxy-exsolution of ulvöspinel from Ti-rich magnetite, whereas nanoinclusions of other minerals likely formed by local supersaturation in the fluid boundary layer, followed by magnetite crystal entrapment. Compositional zoning in magnetite likely formed by a self-organization process where fluid composition fluctuations are feedback responses for an evolving fluid system far from equilibrium, rather than cyclic variations in external factors such as temperature and oxygen fugacity. Saturation of silicate minerals results in relative depletion of Si, Ca, and Al in boundary layer fluids, and formation of inclusion-poor zone depleted in these elements. Nanoinclusions in magnetite highlight the importance of textural characterization when using in situ chemical composition to discriminate the origin of magnetite. In addition, the assemblages of nanoinclusions in magnetite can be used to complement the discrimination of magnetite origins.

不同地质环境的磁铁矿普遍存在成分分带，但其形成机制仍存在争议。在这里，我们使用电子探针微量分析仪 (EPMA) 和透射电子显微镜 (TEM) 表征了来自 Sossego 氧化铁-铜-金矿床（Carajás，巴西）的分区磁铁矿的微米到纳米级结构和化学变化。缺乏孔隙度和微米和纳米尺度的反应前沿表明磁铁矿中的成分分带是晶体生长过程中形成的原始织构，而不是由于溶解和再沉淀反应而形成的二次织构。TEM 能量色散 X 射线光谱分析和绘图确定了分区磁铁矿中的四种纳米夹杂物：1) Mg-Fe-Al 硅酸盐，最有可能是角闪石，2) Fe-Ti 氧化物，主要是钛铁矿，3) 辉石，4) 富硅磁铁矿。Sossego 磁铁矿中钛铁矿纳米包裹体的形成可能是由于富钛磁铁矿中 ulvöspinel 的氧溶出，而其他矿物的纳米包裹体可能由流体边界层中的局部过饱和形成，然后是磁铁矿晶体的俘获。磁铁矿中的成分分带可能由自组织过程形成，其中流体成分波动是远离平衡的演化流体系统的反馈响应，而不是温度和氧逸度等外部因素的循环变化。硅酸盐矿物的饱和导致边界层流体中 Si、Ca 和 Al 的相对贫化，并形成这些元素贫乏的贫包裹体区。磁铁矿中的纳米包裹体突出了在使用原位化学成分区分磁铁矿来源时纹理表征的重要性。此外，磁铁矿中纳米包裹体的组合可用于补充磁铁矿来源的鉴别。