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Podiform magnetite ore(s) in the Sabzevar ophiolite (NE Iran): oceanic hydrothermal alteration of a chromite deposit
Contributions to Mineralogy and Petrology ( IF 3.5 ) Pub Date : 2021-05-18 , DOI: 10.1007/s00410-021-01799-0
Alireza Eslami , Benjamin Malvoisin , Fabrice Brunet , Ali Kananian , Wolfgang Bach , Giovanni Grieco , Alessandro Cavallo , G. Diego Gatta

Serpentinite-hosted massive magnetite ore bodies are reported for the first time in the Late Cretaceous Sabzevar ophiolitic belt, northeastern Iran. They show irregular and discontinuous shapes with variable sizes ranging from 30 to 60 cm. Chromian spinel grains are observed within both magnetite ores and host serpentinite. Magmatic chromian spinels, (Cr,Al)-spinel I, with compositions close to (Mg0.6,Fe0.4)(Cr1.2,Al0.75,Fe3+0.05)O4 are preserved in the host serpentinite where they display a porous alteration rim composed of Cr-bearing chlorite and three different spinel-structure minerals: Cr-spinel (Fe0.6,Mg0.4)(Cr1.4,Al0.4,Fe3+0.2)O4, named Cr-spinel II (second generation), magnetite and ferritchromite, nominally FeCr2O4. In the magnetite ore body, no (Cr,Al)-spinel I is found and Cr-spinel II occurs as relict cores surrounded by ferritchromite and magnetite. Detailed X-ray elemental mapping revealed that the 200 μm-thick magnetite rim is composed of two magnetite types with different minor element compositions: the first rim found at the contact with ferritchromite is thin (20 μm; magnetite-I); the thicker outer rim contains numerous Fe-poor and Mg- and Si-rich silicate inclusions (magnetite-II). Observations at the TEM scale allows to identify ferritchromite which occurs as a micrometer-sized rim between Cr-spinel II and magnetite I. Thermodynamic modelling of the phase relationships in the studied Sabzevar serpentinite suggests that Cr-spinel II is produced along with chlorite during a first alteration stage at temperatures between 725 and 575 °C in the course of peridotite-water interactions. A second hydrothermal alteration stage producing ferritchromite and magnetite is inferred from the thermochemical modelling at temperatures < 400 °C under high H2 fugacity. This latter stage corresponds to the serpentinization of the Sabzevar oceanic peridotite and associated podiform chromitite deposit. The two alteration stages are interpreted as the result of the interaction between seawater and oceanic mantle at two different depth ranges in the course of its exhumation. Our thermodynamic calculations and textural relationships revealed that Cr is immobile and Fe is the main element to be transferred to the magnetite ore during alteration processes. Fe possibly originated from direct transport of the Fe2+ produced during olivine dissolution or from the dissolution of nano-sized magnetite grains initially formed in the host serpentinite during early serpentinization. Mass balance calculation reveals significant iron transport at a scale > 10 m during serpentinization.



中文翻译:

Sabzevar蛇绿岩中的Podiform磁铁矿(伊朗东北部):铬铁矿床的海洋热液蚀变

在伊朗东北部晚白垩世Sabzevar蛇绿岩带中,首次报道了蛇纹岩携带的块状磁铁矿矿体。它们显示出不规则和不连续的形状,大小在30到60厘米之间。在磁铁矿和主体蛇纹岩中均观察到铬尖晶石晶粒。岩浆铬铁尖晶石(Cr,Al)-尖晶石I的成分接近(Mg 0.6,Fe 0.4)(Cr 1.2,Al 0.75,Fe 3+ 0.05)O 4保留在主体蛇纹石中,在那里它们表现出多孔变化含铬亚氯酸盐和三种尖晶石结构矿物组成的轮辋:铬尖晶石(Fe 0.6,Mg 0.4)(Cr1.4,Al 0.4,Fe 3+ 0.2)O 4,命名为Cr-spinel II(第二代),磁铁矿和亚铁铬铁矿,标称FeCr 2 O 4。在磁铁矿矿体中,未发现(Cr,Al)-尖晶石I,而Cr-尖晶石II则是被铁素体铬铁矿和磁铁矿包围的遗迹核心。详细的X射线元素映射显示,厚度为200μm的磁铁矿边缘由两种类型的磁铁矿组成,具有不同的次要元素组成:与亚铁铬铁矿接触时发现的第一个边缘很薄(20μm; I型磁铁矿);较厚的外缘包含许多贫铁,富镁和富硅的硅酸盐夹杂物(磁铁矿-II)。在TEM规模上的观察可以识别出铁铬铁矿,其以Cr-尖晶石II和磁铁矿I之间的微米级边缘出现。在研究的S​​abzevar蛇纹石中的相关系的热力学模型表明,在橄榄岩与水相互作用的第一个蚀变阶段中,在725至575°C的温度下,Cr-尖晶石II与亚氯酸盐一起生成。从热化学模型推断出第二个水热蚀变阶段会产生亚铁铬铁矿和磁铁矿,温度高至<400°C2逸度。后一个阶段对应于Sabzevar大洋橄榄岩的蛇纹石化作用和相关的豆状铬铁矿沉积物。这两个蚀变阶段被解释为海水和海洋地幔在挖掘过程中在两个不同深度范围内相互作用的结果。我们的热力学计算和结构关系表明,在蚀变过程中Cr是固定的,Fe是要转移到磁铁矿中的主要元素。Fe可能源自Fe 2+的直接运输在橄榄石溶解过程中产生的产物,或在早期蛇纹石化过程中最初在主体蛇纹石中形成的纳米级磁铁矿颗粒的溶解产生的产物。物料平衡计算显示,在蛇纹石化过程中,铁的运输在大于10 m的范围内有效。

更新日期:2021-05-18
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