Apatite-monazite relations in the Kiirunavaara magnetite-apatite ore, northern Sweden
Apatite-monazite relations in the Kiirunavaara magnetite-apatite ore, northern Sweden
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DOI：
10.1016/s0009-2541(02)00148-1
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发表时间：
2002-11
期刊：
影响因子：
3.9
通讯作者：
D. Harlov;Ulf B. Andersson;H. Förster;J. Nyström;P. Dulski;C. Broman
中科院分区：
文献类型：
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作者：
D. Harlov;Ulf B. Andersson;H. Förster;J. Nyström;P. Dulski;C. Broman
瑞典北部基律纳地区的磁铁矿-磷灰石矿石通常被认为是早元古代次火山-火山环境中形成的岩浆成因。人们认为它们是从富含挥发物的氧化铁岩浆中结晶出来的，而这些岩浆是通过与钙碱性至微碱性母岩浆不混溶而产生的。使用透射光、背散射电子成像 (BSE)、电子微探针分析 (EMPA) 和激光烧蚀电感耦合等离子体质谱 (LA-ICPMS) 研究了磁铁矿 - 磷灰石矿石的三种主要形态类型（原生矿、角砾岩矿和带状矿）的结构关系和矿物化学。在所有三种类型中，贫 Th 和 U 独居石均以小夹杂物形式存在于磷灰石中。角砾状和带状样品中存在较大（最大 150 μm）的再结晶独居石晶粒，既沿着磷灰石晶界，又与磁铁矿和硅酸盐矿物共生。不含独居石夹杂物的原生磷灰石颗粒通常富含轻稀土元素 (LREE) 以及 Na 和 Si。岩石学和矿物学证据表明基律纳磁铁矿-磷灰石矿石经历了流体-岩石相互作用的连续阶段。第一阶段发生在矿浆就位和结晶后不久的高温条件下（700-800°C），涉及从岩浆中排出的浓缩的、可能以氯为主的盐水。由于 Na 和 Si 的高温浸出，这种流体被认为是磷灰石内大量小独居石夹杂物成核的原因，而轻稀土则集中在独居石中。角砾状和带状样品中的大独居石晶粒被认为是较小的独居石包裹体再结晶的产物。在绿片岩相变质作用期间（T=300–400 °C），来自周围围岩的流体引起沿磷灰石晶界和磷灰石裂纹的强烈（LREE+Na+Si）损耗。轻稀土要么以独居石颗粒的形式沿磷灰石晶界重新沉积，要么从矿石中冲出。这种流体相互作用还引入了硅酸盐组分，这些硅酸盐组分负责沿着磷灰石晶界形成铝铜矿、滑石、透闪石、绿泥石、蛇纹石、白云母、石英和碳酸盐的间隙形成。
The magnetite–apatite ores in the Kiruna area, northern Sweden, are generally considered to be of magmatic origin formed in a subvolcanic–volcanic environment during the early Proterozoic. They are thought to have crystallised from volatile-rich iron oxide magmas derived by immiscibility in calc-alkaline to slightly alkaline parental magmas. Three major morphological types of the magnetite–apatite ore (primary, brecciated, and banded) have been investigated for textural relations and mineral chemistry using transmitted light, back-scattered electron imaging (BSE), electron microprobe analysis (EMPA), and laser ablation–inductively coupled plasma-mass spectrometry (LA–ICPMS). In all three types, Th- and U-poor monazite is present as small inclusions in the apatite. Larger (up to 150 μm) recrystallised monazite grains, both along apatite grain boundaries and intergrown with magnetite and silicate minerals, are present in the brecciated and banded samples. Primary apatite grains, without monazite inclusions, are generally enriched in light rare earth elements (LREEs) together with Na and Si. Petrological and mineralogical evidence suggest that the Kiruna magnetite–apatite ore experienced successive stages of fluid–rock interaction. The first stage occurred under high-temperature conditions (700–800 °C) shortly after emplacement and crystallisation of the ore magmas and involved concentrated, probably Cl-dominated brines expelled from the magma. This fluid is held to be responsible for the nucleation of the numerous small monazite inclusions within the apatite due to high-temperature leaching of Na and Si, while the LREEs were concentrated in the monazite. The large monazite grains in the brecciated and banded samples are proposed to be the product of recrystallisation from the much smaller monazite inclusions. During greenschist-facies metamorphism (T=300–400 °C), fluids from the surrounding country rock caused strong (LREE+Na+Si) depletion along apatite grain boundaries and cracks in the apatite. LREEs were either redeposited as monazite grains along apatite grain boundaries or were flushed out of the ore. This fluid interaction also introduced the silicate components responsible for the interstitial formation of allanite, talc, tremolite, chlorite, serpentine, muscovite, quartz, and carbonates along apatite grain boundaries.