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首页> 外文期刊>Contributions to Mineralogy and Petrology >Graphite morphologies from the Borrowdale deposit (NW England, UK): Raman and SIMS data
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Graphite morphologies from the Borrowdale deposit (NW England, UK): Raman and SIMS data

机译：Borrowdale矿床（英国西北英格兰）的石墨形态：拉曼和SIMS数据

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Graphite in the Borrowdale (Cumbria, UK) deposit occurs as large masses within mineralized pipe-like bodies, in late graphite–chlorite veins, and disseminated through the volcanic host rocks. This occurrence shows the greatest variety of crystalline graphite morphologies recognized to date from a single deposit. These morphologies described herein include flakes, cryptocrystalline and spherulitic aggregates, and dish-like forms. Colloform textures, displayed by many of the cryptocrystalline aggregates, are reported here for the first time from any graphite deposit worldwide. Textural relationships indicate that spherulitic aggregates and colloform graphite formed earlier than flaky crystals. This sequence of crystallization is in agreement with the precipitation of graphite from fluids with progressively decreasing supersaturation. The structural characterization carried out by means of Raman spectroscopy shows that, with the exception of colloform graphite around silicate grains and pyrite within the host rocks, all graphite morphologies display very high crystallinity. The microscale SIMS study reveals light stable carbon isotope ratios for graphite (δ 13C = −34.5 to −30.2‰), which are compatible with the assimilation of carbon-bearing metapelites in the Borrowdale Volcanic Group magmas. Within the main mineralized breccia pipe-like bodies, the isotopic signatures (with cryptocrystalline graphite being lighter than flaky graphite) are consistent with the composition and evolution of the mineralizing fluids inferred from fluid inclusion data which indicate a progressive loss of CO2. Late graphite–chlorite veins contain isotopically heavier spherulitic graphite than flaky graphite. This agrees with CH4-enriched fluids at this stage of the mineralizing event, resulting in the successive precipitation of isotopically heavier graphite morphologies. The isotopic variations of the different graphite morphologies can be attributed therefore, to changes in the speciation of carbon in the fluids coupled with concomitant changes in the XH2O during precipitation of graphite and associated hydrous minerals (mainly epidote and chlorite). Keywords Graphite - Morphology - Raman - Carbon isotopes - Borrowdale Communicated by J. L. R. Touret.
机译：Borrowdale（英国坎布里亚郡）矿床中的石墨以大块状存在于矿化的管状物体中，晚于石墨-亚氯酸盐矿脉中，并通过火山岩层扩散。这种现象显示出迄今为止从单一沉积物中识别出的最大种类的结晶石墨形态。本文描述的这些形态包括薄片，隐晶和球状聚集体以及碟状形式。许多隐晶聚集体显示的共形纹理首次在全球范围内的任何石墨沉积物中被报道。纹理关系表明，球状聚集体和共形石墨比片状晶体形成得早。该结晶顺序与随着流体的过饱和度逐渐降低而从流体中沉淀出石墨相吻合。用拉曼光谱法进行的结构表征表明，除了基质岩中硅酸盐颗粒周围的共形石墨和黄铁矿外，所有石墨形态都显示出很高的结晶度。微型SIMS研究揭示了石墨的光稳定碳同位素比（δ 13 C = -34.5至-30.2‰），与Borrowdale火山群岩浆中含碳变质岩的同化作用兼容。在主要矿化角砾岩管状物体中，同位素特征（隐晶石墨比片状石墨轻）与根据流体包裹体数据推断的矿化流体的组成和演化相一致，这表明CO 2逐渐损失。后期的石墨-亚氯酸盐矿脉比片状石墨含有同位素重的球状石墨。这与矿化事件这一阶段富集CH 4 的流体是一致的，从而导致同位素重石墨形态的连续析出。因此，不同石墨形态的同位素变化可以归因于流体中碳形态的变化，以及石墨和相关含水矿物（主要是附生矿物）沉淀过程中XH 2 O的伴随变化。和亚氯酸盐）。关键字石墨-形态-拉曼-碳同位素-Borrowdale由J. L. R. Touret沟通。

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