Hydrothermal rare earth element (REE) precipitation in carbonatite is generally attributed to increase in pH or decrease in temperature when REE-bearing acidic fluids interact with carbonates. Here, we document microtextures comprising intergrowth of bastnäsite, hydroxyl-parisite, röntgenite, synchysite, and pyrochlore, with calcite that pseudomorphically replaces patches of hematite with cellular boxwork-type structure, in calciocarbonatites from the Kamthai alkaline complex in western India. The nature of the REE mineralization grades from proximal bastnäsite-dominated to distal hydroxyl-parisite dominated in the boxworks. The microtextural relations and trace element chemistry of hematite, magnetite and calcite, and C-O isotope composition of carbonate are suggestive of extensive low-temperature hydrothermal alteration of the carbonatites. The hematite boxwork structure and the REE mineral-calcite intergrowths often have squarish outlines and are interpreted to have pseudomorphed primary magnetite during fluid-rock interaction. We propose a new mechanism of REE precipitation in magnetite-rich carbonatites involving influx of acidic hydrothermal fluids, which scavenged the REE and other trace elements from magmatic carbonates and apatite. These acidic fluids were responsible for the protonation of magnetite and leaching out of Fe, converting them to hematite through a non-redox transformation. The reaction results in 32% volume reduction for every mole of magnetite consumed, generating significant rock porosity, which further aided and abetted fluid-rock reaction and hydrothermal alteration. More importantly, it consumed proton, which increased the fluid pH triggering precipitation of bastnӓsite-group minerals (including bastnӓsite, parasite, röntgenite, synchysite). Close to the magnetite-hematite reaction front, fluor-dominated bastnӓsite-group minerals (mainly bastnӓsite) appear, while away from such fronts, mineralization was enriched in hydroxyl-bastnӓsite group minerals (mainly parisite) as a consequence of decreasing activity of F and/or increasing activity of OH with the progress of magnetite-to-hematite transformation.

中文翻译：

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磁铁矿非氧化还原转化为赤铁矿引起的稀土元素沉淀：来自印度西部 Kamthai 碳酸岩​​杂岩的显微结构和地球化学证据

碳酸岩中的热液稀土元素 (REE) 沉淀通常归因于含 REE 的酸性流体与碳酸盐相互作用时 pH 值升高或温度降低。在这里，我们记录了来自印度西部 Kamthai 碱性杂岩的碳酸钙岩中的微观结构，其中包括氟碳铈矿、羟基磷灰石、菱铁矿、正晶石和烧绿石与方解石的共生，方解石假象地取代了具有蜂窝箱型结构的赤铁矿斑块。稀土矿化等级的性质从近端氟碳铈矿为主到远端羟基磷灰石在箱体中占主导地位。赤铁矿、磁铁矿和方解石的微观结构关系和微量元素化学以及碳酸盐的CO同位素组成表明碳酸岩发生了广泛的低温热液蚀变。赤铁矿盒状结构和稀土矿物-方解石共生体通常具有方形轮廓，并被解释为在流体-岩石相互作用过程中具有假象的原生磁铁矿。我们提出了一种在富含磁铁矿的碳酸岩中稀土元素沉淀的新机制，涉及酸性热液的流入，从岩浆碳酸盐和磷灰石中清除稀土元素和其他微量元素。这些酸性液体负责磁铁矿的质子化和铁的浸出，通过非氧化还原转化将它们转化为赤铁矿。该反应导致每消耗一摩尔磁铁矿，体积就会减少 32%，产生显着的岩石孔隙度，这进一步帮助和促进了流体岩石反应和热液蚀变。更重要的是，它消耗质子，从而增加流体 pH 值，引发氟碳铈矿族矿物（包括氟碳铈矿、寄生虫、菱锰矿、合质石）的沉淀。靠近磁铁矿-赤铁矿反应前沿，出现以氟为主的氟碳棒铁矿族矿物（主要是氟碳棒铁矿），而远离这些前沿，由于 F 和 活性降低，矿化富集在羟基氟碳棒铁矿族矿物（主要是钙铁矿）中。 /或随着磁铁矿向赤铁矿转化的进行，OH的活性增加。