Liu Yan, Anenburg Michael
State Key Laboratory of Deep Earth and Mineral Exploration, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, PR China.
Research School of Earth Sciences, Australian National University, Canberra, Australia.
Nat Commun. 2025 Aug 4;16(1):7159. doi: 10.1038/s41467-025-62009-0.
Igneous rocks form by solidification of magmas through cooling or volatile degassing following decompression. Expelled HO is thought to trigger alteration around intrusions, leading to formation of metasomatic halos. This mechanism is often invoked to explain many magmatic-hydrothermal rock associations, some of them economically mineralised. Maoniuping in China is one of the four largest operating rare earth element (REE) mines globally, whose origin has been attributed to such hydrothermal exsolution. However, no direct evidence links hydrothermal fluids to the formation of Maoniuping and its associated REE mineralisation. Here we show that the REE deposit at Maoniuping formed magmatically from a carbonatitic brine-melt. Textural and chemical evidence reveals extensive interaction with its quartz syenite host, producing albitised fenites. Coupled metasomatism with these fenites led to silica contamination of the carbonatite melt, triggering crystallisation of refractory alkali-ferromagnesian silicates-an antiskarn. This solidified the melt due to removal of the fluxing elements Na and K. Thus, carbonatite melts can crystallise by element assimilation from their environments, precipitating alkali liquid fluxes into solid minerals. Temperature decrease and volatile degassing merely play a secondary role in this igneous rock-forming process. Solidification driven by coupled antiskarnisation and fenitisation affects both the mineral assemblage and ore fabric, and likely operated in most carbonatite-hosted REE deposits elsewhere.
火成岩通过岩浆冷却或减压后挥发分脱气固化形成。排出的水被认为会引发侵入体周围的蚀变，从而形成交代晕。这种机制常被用来解释许多岩浆热液岩石组合，其中一些具有经济矿化价值。中国的牦牛坪是全球四大正在运营的稀土元素（REE）矿山之一，其成因被归因于这种热液溶出作用。然而，尚无直接证据将热液流体与牦牛坪的形成及其相关的REE矿化联系起来。在此我们表明，牦牛坪的REE矿床是由碳酸岩盐水熔体通过岩浆作用形成的。结构和化学证据揭示了其与石英正长岩围岩的广泛相互作用，形成了钠长石化云英岩。与这些云英岩的耦合交代作用导致碳酸岩熔体受到硅污染，引发难熔碱铁镁硅酸盐（一种反矽卡岩）的结晶。由于助熔元素钠和钾的去除，熔体得以固化。因此，碳酸岩熔体可通过从其环境中同化元素而结晶，将碱性液体助熔剂沉淀到固体矿物中。在这个火成岩形成过程中，温度降低和挥发分脱气仅起次要作用。由反矽卡岩化和云英岩化驱动的固化作用影响了矿物组合和矿石结构，并且可能在其他地方大多数以碳酸岩为宿主的REE矿床中都有发生。
