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首页> 外文期刊>Mineralogical Magazine >The magnetite ore districts of the southern Aosta Valley (Western Alps, Italy): a mineralogicai study of metasomatized chromite ore
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The magnetite ore districts of the southern Aosta Valley (Western Alps, Italy): a mineralogicai study of metasomatized chromite ore

机译：瓦莱达奥斯塔南部（意大利西阿尔卑斯山）的磁铁矿区：交代铬铁矿的矿物学研究

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In the southern Aosta Valley (Italian Western Alps), several massive magnetite bodies occur within serpentinized ultramafic rocks belonging to the Mesozoic meta-ophiolite nappe. The ultramafic rocks consist of lherzolite with minor dunite bodies and show a high pressure metamorphic overprint. The results of a multi-methodological study, based on optical microscopy, electron microprobe analysis and single-crystal X-ray diffraction, are reported here in order to give new insights into (1) the mineralogy and crystal chemistry of spinels and silicates and (2) the genesis of the massive magnetite bodies. Chromium-rich relict cores inside the magnetite grains suggest a derivation from primary chromite concentrations. The major-element behaviour shows the presence of two chromite types: a Cr_2O_3-rich (Al_2O_3-poor) type and a Cr_2O_3-poor (Al_2O_3-rich) type. Magnetite ore deposits probably represent the product of transformation from a chromite proto-ore which formed in ultramafic rocks pertaining to an ophiolite suite. The transformation of chromite to magnetite occurred during multiple stages: the pre-metamorphic setting of the ultramafics and the petrographic evidence suggest that metasomatism started before the onset of the alpine metamorphism and was active during the early alpine, eclogite-facies metamorphic overprint related to a subduction process under high fluid activity.
机译：在奥斯塔山谷南部（意大利西部的阿尔卑斯山），蛇纹石化的超镁铁质岩石中出现了几块块状磁铁矿体，这些岩石属于中生代蛇纹石推覆岩浆。超镁铁质岩石由水辉石和少量的榴辉岩组成，并显示出高压变质叠印。此处报告了基于光学显微镜，电子探针分析和单晶X射线衍射的多方法研究的结果，以便对（1）尖晶石和硅酸盐的矿物学和晶体化学以及（ 2）块状磁铁矿的成因。磁铁矿晶粒内部富含铬的残余铁心暗示了原始铬铁矿的浓度。主要元素行为表明存在两种亚铬铁矿类型：一种富含Cr_2O_3-（贫Al_2O_3-）和一种富含Cr_2O_3-（贫Al_2O_3-）。磁铁矿矿床可能是铬铁矿原矿转化的产物，铬铁矿原矿形成于与蛇绿岩组有关的超镁铁质岩石中。铬铁矿向磁铁矿的转变发生在多个阶段：超镁铁矿的前变质环境和岩石学证据表明，交变作用在高山变质作用发生之前就开始发生，并且在早期的榴辉岩相变质叠印过程中活跃。高流体活动下的俯冲过程。

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