Hydrothermal processes in the Fen alkaline-carbonatite complex, southern Norway
Hydrothermal processes in the Fen alkaline-carbonatite complex, southern Norway
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DOI：
10.1016/j.oregeorev.2019.102969
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发表时间：
2019-08-01
影响因子：
3.3
通讯作者：
Markl, Gregor
中科院分区：
文献类型：
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作者：
Dietzel, Christian A. F.;Kristandt, Tim;Markl, Gregor
我们提出了详细的结构和化学分析的热液硫化物和REE-Th-Nb矿化观察芬复杂（挪威南部），这是最大的碳酸盐岩相关的稀土和Th存款在欧洲。该杂岩的碱性硅酸盐岩和碳酸岩经历了两次热液蚀变事件，导致硫化物的形成和稀土、铁、铝的再活化，是研究碳酸岩中硫化物和稀土成矿作用的理想场所。我们的观测记录了汾河碳酸盐岩杂岩的热液蚀变历史，这与世界上许多碳酸盐岩杂岩有关。第一次蚀变事件（黄铁矿阶段）引起了所有岩性中硫化物（主要是黄铁矿）的结晶，并由可能来自邻近镁铁质碱性硅酸盐岩的富硫化物流体1引起。该热液事件形成的脉体具有典型的磁铁矿→黄铁矿I +赤铁矿+磁铁矿→黄铁矿II的演化过程。即（1）可能来自碳酸岩的富REE流体2（2）以引入Si为主并与基底容矿岩处于平衡状态的氧化性大气流体3。这两种流体与碳酸盐岩的相互作用导致了各种典型的方解石-赤铁矿岩石，当地称为罗德贝格。在后期流体3的硅化作用中，P从岩浆磷灰石中被活化，并在附近的罗德贝格脉中再沉淀为氟磷灰石-独居石脉，碳酸盐岩衍生流体2和大气流体3的相互作用也导致了LREE相的形成，特别是REE-F-碳酸盐（氟碳铈矿、氟碳铈矿、氟碳铈矿）、独居石和褐帘石以及HREE相（包括钐铁铈矿、易解石和未命名的Nb-Fe-REE-Th-氧化物）。最明显的HREE富集发生在最强的热液蚀变，这可能是由于优先沉淀的HREE丰富的配合物，再加上后来的部分浸出LREE富F流体在硅化的域。富钍矿物与富HREE矿物密切相关，这意味着Th和HREE的热液行为相似。
We present detailed textural and chemical analyses of the hydrothermal sulfide- and REE-Th-Nb-mineralization observed in the Fen complex (southern Norway), which is the biggest carbonatite-related REE and Th deposit in Europe. The alkaline silicate rocks and carbonatites of the Fen complex underwent two hydrothermal alteration events that caused (i) the formation of sulfides and (ii) (re)-mobilization of REE, Fe and Al. This renders the Fen complex an ideal locality to study the genesis of hydrothermal sulfide and REE mineralization in carbonatites. Our observations record a hydrothermal alteration history of the Fen carbonatite complex, which is relevant to many carbonatite complexes worldwide.The first alteration event (pyrite-stage) caused the crystallization of sulfides (mostly pyrite) in all lithologies and was induced by a sulfide-rich fluid 1 that was probably derived from adjacent mafic alkaline silicate rocks. Veins formed during this hydrothermal event show a typical succession from magnetite via pyrite I + hematite + magnetite to pyrite II.A subsequent alteration event is characterized by the interaction of two evolving fluids, namely (1) a REE-rich fluid 2 that was probably derived from the carbonatites (autometasomatic fluid) and (2) an oxidizing meteoric fluid 3 that mainly introduced Si and was in equilibrium with the basement host rocks. The interaction of these two fluids with the carbonatitic rocks resulted in various characteristic types of calcite-hematite rocks locally called rodberg. During their late-stage silicification by fluid 3, P was mobilized from magmatic apatite and reprecipitated in close-by rodberg veins as small fluorapatite-monazite veins.The interaction of the carbonatite-derived fluid 2 and meteoric fluid 3 also resulted in the formation of distinct zones of LREE-phases, notably REE-F-carbonates (bastnasite, parisite, synchysite), monazite and allanite, and HREE-phases (including samarskite, aeschynite and an unnamed Nb-Fe-REE-Th-oxide. The most pronounced HREE-enrichment occurs in domains of strongest hydrothermal alteration, which was probably caused by preferential precipitation of HREE-rich complexes coupled with later partial leaching of LREE by F-rich fluids during silicification. Thorium-rich minerals were precipitated in close association with HREE-rich minerals implying similar hydrothermal behaviour of Th and HREE.